This is indeed a really rare and unusual example of the WW2 German MP-40 SMG. This submachine gun has a distinctive enlarged magazine housing that takes two MP-40 magazines and allows switching between those in a matter of pressing a latch in front of the magwell and sliding the two side by side oriented magazines left or right.
The idea was to have an ability to faster resume firing when the magazine was empty compared to changing the magazine in a standard MP-40. This sample is one of the three MP-40/I SMGs registered in the USA.
