Just a SWAG..... some type of flare pistol?
I remember seeing a "starter pistol" years ago that looked a little similar. It was made to look like a .25 auto pistol (Rohm?) but the "magazine" was a small rectangular block that was inserted lengthwise into the guns frame below the barrel. It was drilled to hold 6 .22 cal blank shorts and would ratchet through the frame after each shot, and come out the other end when all the rounds were fired. The blanks faced upward toward the barrel and I assume fired through a port in the bottom of the barrel.
Perhaps this is a sort of adaptation to fire (4) flares in the rack on top of this frame? I dunno......