Sights and Predictor computers on the Bofors 40mm, Mk I, of European Manufacture

[Old Git]

Before we start, you may to get a cup of coffee, or even a pot, this is going to be a long one!

Trying to get my head around the sighting systems, (i.e. the 'Bofors Course and Speed Corrector' with optical sights) on the early Mk I 40mm Bofors guns that were made in Sweden, Belgium, Poland, Hungary and possibly Finland.

From the research I've already done I understand that there were three different sighting systems used on the early models. Firstly all the guns used Optical reflex mirror sights, (which in British parlance were known as the "Polish" sights). These were attached to a Predictor, or target calculator (a type of analog computer), which was attached to the right side of the breech. So let's deal with this in two parts, Optical Sights first and then the Predictor Units.

1.) From what I can understand there were at least two different types of Optical sights: the Agfa-Baltic Optical Sights and the Zeiss Optical Sights.

i.) The Agfa-Baltic Optical Sights.

Below are what I believe are a period drawing and photograph of these particular sights. The first thing to notice from the drawing is that this sight is a fairly straight oblong box with mirrors configured internal as in an old trench periscope. Also note the additional, cage-like sight attached to the side of the the main box sight. How these two worked in practice I have yet to figure out!

 

 

 

 

These appear to be same as the Optical Sights that appeared on Polish guns, as can be seen in this drawing.

 

 

And here is a photo of a Polish Bofors in use in 1939

 

Here's a cropped, close-up of the area in question.

 

 

These same sights can be seen on a Finnish gun now owned by Rob Fast in Canada.

 

 

 

 

ii.) The Zeiss Optical Sights.

 I've found much for the Zeiss sights, so far, however, in this drawing you can see that these Optical sights differed in a number of ways from the Agfa-Baltic optical sights. The most significant being that the sight boxes containing the mirrors are no longer a straight, rectangular box. There is now a very noticeable angling effect at the front of the sight box. The cage-like secondary sight has also been modified to something more akin to iron sights on a rifle.

 

 

 

This difference in box-styles for the optical sight can be clearly distinguished on this Bofors in use with some modern Polish re-enactors
 

 

Here's a cropped, close-up of the main area of the sights

 

 

Compare this to this 1939 photo of a Polish crew and right away you can see the difference between what appears to be an Agfa-Baltic sight below and a Zeiss optical sight above

 

 

Finally, here are a couple of period photographs showing the two different optical sights mounted on different guns. 

 

 

 

And just to confuse the narrative a wee bit, I've come across this period photograph of a Bofors in Swedish service, pre-war. As you can see this gun is using the Zeiss Optical Sights, however it appears to have the secondary, cage-like sights on the side as opposed to the 'iron sights' shown in the drawing above. So, for identification purposes we are reliant upon shape and style of the housing for the Optical Sights rather than any attachments on the side of Optical Sight. 

 

 

So, as far as I have been able to ascertain, that is it for pre-war Optical Sights on the 40mm Bofors that were manufactured in Sweden, Poland, Hungary and Belgium. Have I got this correct so far? Is anyone aware of any other type of Optical Sights? Feedback and Observations are most welcome. I'll add another post in just a moment covering the Predictor Units.

[Old Git]

2.) The Predictor Unit, an Analog Computer known more formally as the 'Bofors Course and Speed Corrector'

From what I can figure out, there were three different types of Predictor Units, and these were as follows:

1. Belgian or Swedish made     = Bofors, M/35 FC
2. Polish made Bofors                = Goerz,  M/36 FC
3.  Hungarian made Bofors      = Johasz-Gamma Loelemkepzo M 34/38

These predictors were similar in design but did have notable differences. So far I think I have indentifed the differences between the Bofors M/35C and the Johasz-Gamma M/38 FC, which is what I intend to discuss below. I'd be grateful for any input\corrections that that others might be able to offer?

As far as I can ascertain the drawing below shows the standard Bofors M/35 FC Predictor that was shipped on Swedish and Belgian manufactured Bofors 40mm in the mid to late 1930s.  

 

Here's a second copy of that drawing with two points marked for discussion on how the Bofors, M/35 FC differed from the Hungarian Johasz-Gamma Loelemkepzo M 34/38 Predictor.

 

Point A, on the Bofors M/35 FC Predictor these are located on a vertical face, and this can be clearly seen in the next couple of images.

Point B. Is the Azimuth adjustment knob and on the Bofors, M/35 FC there is nothing else below this point, and no other additions protruding back towards the operator, this can be discerned on the photos above. There is just a round housing with the Azimuth knob below that.

So, that's what these two areas look like on the Bofors M/35 Predictor. On the (what I believe to be) the Johasz-Gamma, M/38 FC the dials at Point A are slightly raised up, out of the body of the Predictor, and laid backwards onto a 45 degree face that points upwards towards the operators eye-line, presumably to make life a little easier for the operator. You will see this quite clearly on the period image below, and on the cropped, close-up.

 

 

Note also, that where the Azimuth knob was on the Bofors M35 FC, there is now an additional assembly which projects backwards towards the operator and presents a flat, circular surface with a disc control unit, which presumably is where the Azimuth control has been re-located. Again, one presumes an ergonomic design to make life easier for the operator.

When Rob Fast first received his Bofors it had the same configuration, see photo below where you can just make out the flat dial protruding from behind the back-rest of the operators chair. 

And here are some photographs of a dismounted Gamma M38 Predictor from a Bofors 40mm in Argentina

 

 

 

Note that this new assembly does seem to be a solid piece of the Predictor, and almost looks as though it is cast as one piece with the body of the Predictor. Which of course it cannot be as it would probably be counter-productive to have this dial move away from the operator when the Predictor moved with the gun. Also, Rob Fast would not have been able to have modified his Predictor by removing this assembly when he undertook his restoration, see images of Rob's Predictor unit below.

 

 

A third area of difference between these two predictors would appear to be Point C, see a familiar image below of a Bofors M35 Predictor.

 

And below is a cropped, close-up of the same area. You can clearly see that coming out of the top of the Predictor Unit there is a rod with a hinged assembly on top, which appears as it should opens and close as the rod goes up and down (if the rod does actually go up and down). If you look at the first drawing you'll see this is called the 'Corrector Resultant Transmitting Screw' and as it does not go inside the predictor, but is instead connected to a sliding plate on top of the predictor, then I cannot see how it would move up and down, so I do wonder if the hinged contraption is simply to add strength and stability to an area constantly under stress of movement.  

Attached to the top of this CRT Screw, and projecting away at a 45 degree angle, is a second rod which is connected/goes into a squared-shaped unit that is bolted to the gun receiver, via a long tube. To the front-face of this square-shaped box there are two more, hinged, rods, one top and one bottom which lead off to connect to the bar holding the Optical Sights. Between both of these hinged rods is a knurled knob which I presume is for tightening the movement of the rod coming from the top of the CRT Screw on the Predictor unit.

 

The next photograph shows a Bofors 40mm housed in a museum, somewhere in Finland I think, and it has, what appears to be, a Bofors M35 Predictor attached to this particular square-shaped box assembly. This is the best photograph I can find of this assembly for the basis of this discussion. I said it appears to be an M35 but if you closely enough at the rear end of the Predictor you will see that the Point A dials do in fact seem to be raised and laid back in a 45% angle, which would make this a Gamma M38 Predictor. Note that it does not have the Point B addition either. It's a Museum piece so we might assume that over time it has been restored with what ever pieces could be found?

 

You can also see, in the photo above, how the Pedictor unit begins to rotate outwards as the gun is elevated.

On the Gamma M38 Predictor, the Square-shaped box has been replaced by a large bulbous contraption, which basically does the same job. It can be clearly seen here in this photograph of the pre-war Bofors held by the Shuttleworth collection.

 

The next two photographs are of the same gun and you can clearly see the Point A dials laid at a 45 degree angle and the manner in which the Optical Sights bar is connected to this particular unit.

 

 

The next two photographs are also the Shuttleworth collection gun and you can clearly see the Point C additional assembly for the Azimuth controls.

 

 

Last two photographs are of the gun in private hands in Argentina. First photos shows the large bulbous item unattached to the Predictor. Whilst the second photograph shows some close-up detail of the Predictor unit refitted to the gun.

 

 

Finally just found this superb photo of what  I think is the Hungarian-made Johasz-Gamma Loelemkepzo M 34/38, which is just too good not to included it here for reference.

 

That's what I think I've worked out about these units and just wondered if anyone has any comments to make? Correct my glaring mistakes? Provide correct nomenclature for the various parts?

Does anyone have anything to add on what the Polish Goerz M/36 FC Predictor looks like in comparison to Swedish and Hungarian Predictors (albeit that I have indeed correctly identified these two sights)?

Would anyone care to speculate if specific Optical Sights were used with specific Predictor units, or was it a simple case of Mix n match?  Any thoughts at all folks?