Friend of mine found this recently among his grandfather's things... He messaged me some pics from an iPhone.
From what I can gather it's a 27mm Walther 'Doppel Schuss' no. 912/N440 Signal Pistol, manufactured 1939 - probably for the Kriegsmarine. I'll include the pictures sent to me below. Appears to be stainless; definitely responds to magnets.
I'm looking for confirmation/insight/any other information you have on the piece, or at the very least perhaps someone could point me to someone that might know more. Thanks in advance.