German MP 28/II submachine gun, an improved version of the MP 18 submachine gun with fire selector and tangent sight. Both designed by Hugo Schmeisser. Feeds from 20-round box magazines (or 32 round artillery Luger snail drums if chambered in 9mm Luger) and was mostly produced under a number of foreign licensed companies over a veritable rainbow of calibers from 9mm Luger to 7.63X25 Mauser to .45 ACP.
This particular picture depicts a specimen made by the C.G. Haenel Company in Suhl, Germany right before the onset of World War II.