This gun appears to be the oldest of the four old target Dianas that I have. The stock details are subtly different from the 50/b, especially around the grip and comb nosing. And the trigger details are again different, note the use of the tiny pre-war style adjuster screws and different location of the adjuster access hole in the trigger guard (this album has a shot of the 50/b trigger for comparison).
All that's needed to turn a "50/a" into a "50/b" is the clamp-on peep disk of course! Otherwise the actions of the a, b, and M are identical.