There is very little documentation existing to explain the history of this rifle – all we really know is that per the receiver markings it is a Model 1936 Stendebach, and that it was brought back from Bavaria in 1945 by a US soldier who found it in a collection of confiscated firearms.
A number of German patents exist granted to one Friedrich Stendeback from 1913 to the mid 1920s, covering aspects of a toggle-locked rifle like this one. It is a delayed blowback system, with neither a recoiling barrel nor any sort of gas system. The rotary magazine can be fed by single rounds or 5-round stripper clips.
The aluminum elements of the gun raise some questions, and it seems most likely that the gun was incomplete when acquired in Germany. The firing mechanism does not appear to have been completed, which supports this suspicion. Even so, it is an interesting and unique combination of features. Hopefully more information will come out about Stendebach’s other work and potential German military testing of hist designs!