Mystery solved! And how! The truly bizarre “M17” Frommer Stop Long-Recoil Dual Submachine-gun.
Quite a while ago I came upon a curious photo of a Frommer Stop that had no visible trigger guard or trigger. Some folks on a Russian gun forum had been guessing about it but hadn’t come up with anything definitive.
Imagine my surprise when I discovered that it must have been one of a matched set of Frommer submachine-guns originally meant for mounting on a tripod!
According to various writers on the greatwarforum, the Austro-Hungarian High Command had been inspired by the Italian Villar Perosa twin barreled submachine-gun, primarily because of its extremely high rate of fire - “as fast as 1500 rpm per barrel”. As is shown one result of this inspiration was the creation of a tripod mounted submachine-gun based on the native Hungarian Frommer design, although I’m not sure whether the end result was chambered in .32 or .380 ACP (presumably the former). I wonder if the long-recoil action would have slowed the rate of fire at all as compared to other submachine-guns. Also, each individual pistol can be fired by hand simply by exerting sufficient pressure on the tab just above the grip safety, sometimes causing them to be called “palm guns”.
This arrangement seems to have been considered unsatisfactory as the Austro-Hungarians ended up producing a direct copy of the Villar Perosa called the MP18.
Find out more in “Ortner, M. Christian (2006). Storm Troops: Austro-Hungarian Assault Units and Commandos in the First World War.” (Militaria Verlag).