There's an article in Am. Rifleman, Apr '76, pg53 about this.
I'll pay for a copy of the page if anyone has a copy.

Made in Germany late '20s/mid '30s I think, sn 26xx.
Bbl marked "Cal. 12 - 65m/m = 2 1/2".
Which is correct? I shoot 2 1/2" for safety.

Nitro prooved, with Ulm (?) proofmarks on bbl bottom - crown over W, cross-topped crowns over U & S. Also "13/1", circled "12", inverted script "R" & 3 incomplete Eagle stamps.

But it doesn't operate semi-auto.
Fires but must use cocking lever to eject/reload.
Seems to be short-recoil system, as the bbl/upper rcvr will move rearward ~ 1/2" by hand under very stiff spring pressure.
There's a Luger-like link between the mainspring /bolt which the cocking lever operates upon.
I think this link *should* be breaking but doesn't.

Is anyone familiar with the operating principles of this design?

Any info greatly appreciated.

Thanks.