Japanese SNLF marines Chinese troops with SIG M1920 submachine guns. The SIG M1920 was a licensed copy of the German MP18/I, and SIG sold them to Japan in 7.63mm Mauser caliber. They were used by the Special Naval Landing Forces, as well as Chinese troops like those pictured here. Thanks to Leszek for the photo!
Edited: Based on the comments folks more knowledgeable than I, it appears that these are Chinese puppet troops, and not Japanese forces.