Lucas numbering can get complicated and I'm not fully conversant with them. Some of the pre-war ammeters had a BM number, and at that time in 1939 there was a period of just using civilian stuff for WD ....to modified and purpose made for WD.
Unless you want to get into the research of exactly how your bike might have left the factory, which can sometimes be very difficult as although the parts might have changed, often the part numbers didn't, and it sometimes has to be done with photographic evidence, which of course are usually B&W..... Some of us have been squinting and zooming in for 20-30 years and still never know for sure.
As far as the ammeter is concerned, a straight forward CZ27 which covered the whole of war-time, Lucas equipped bikes will suffice. However I know a guy who can supply or fit a dome glass for a 1939 effect if required.
I have 2 or 3 with a dome glass, but I think only one that has the illumination window. Again I have 2 or 3 with a plated bezel, which would have been fazed out for economy and to speed up supply as war progressed.
Otherwise most of my CZ27's have bare brass bezels as on my 3SW here. Ron