Welcome to Leica Place-
I read the article on the Leica II: remember that the plate for the finder housing for the IIIa is the same as the II. I'll bet the top plate of the original was damaged, and someone replaced it with the one on it now. Not hard to do at all. I've never done that with a Leica, but have done so with Kodak Retinas, Canon RF's, and others. I've repaired a Contax II using parts from a Russian Kiev 4.
And- it might be that your camera was indeed updated with a Rangefinder to a Leica II from a viewfinder-only model. The easiest way to get a top plate for the upgrade would be to pull one from the "assembly line"- no need to re-engrave the original serial number to a blank top plate.