Hello,
Thanks to your replies, I have found another board of the blue tractors: http://www.fordsontractorpages.nl/ubb/Forum2/HTML/000050.html
This Ford has the same axle as we have, so it is a Selene conversion indeed.
I don't have it quite sure at this moment, but what I understand from that topic, is that in those years of building (late 60's / early 70's), Selene used the back ends of the GMC axles and made an own inner section for it.
I have registered there too, to ask the people over there, but it takes some time to get my profile activated.
When I know some more, I'll post it over here.
Thanks and see you!
Best wishes,
Marten