Yes, Jacques, I think your theory is correct. I don't think this one has anything to do with FED and is some variation, probably by a craftsman, camera shop, etc. as you say.
I have a Leica from this time that does have an early FED rangefinder top on it ( FED top # 4042 / chrome). It is Leica II in every way on the body, except it has the FED rangefinder top and a Leica accessory shoe screwed onto that FED top (where there was once none on the FED top). Even the lens is the original Leitz Elmar.
I have often thought that if you lived in the USSR at that time (1930s) and you had a Leica, when it was broken you could probably either fix it yourself with FED parts, have a craftsman fix it with FED parts, or possibly even take it to the FED Zavod (or Trudkomma) to have it fixed with FED parts.
Jacques, I will tell you if I can remember where I saw the diagonal part, but so far ... no.
Regards, Bill
01/26/2008 17:58