It is for sure a Mp 34(O).
Now, by 1934, the Germans where not allowed to use any other submachinegun then Mp 18/28 (versilletreaty). So it was manufactured for Austria and incorperated into the German defence as Austria was annected by Germany This giving it the (O) Osterreich designation. The development of the gun was made in Swiss, by Solothurn, a German owned company.
The Mag. can be loaded or toped up while still inserted to the gun by stripper clips.
Used By The SS and other Policeforces.
Also called "Bergman", by the Swedes who was one of the countries that used it.
(Edit, It was used by coastdefence, because of the side loaded Mag, who allowed the shooter to get closer towards the ground, not giving away to much siluette.)
Most Bergman Guns where constructed by Schmiesser and Most Schmissers where constructed by Bergman.
This piece is on my wishlist, as well as the "Thompson" with Swedish defenceforces markings.