Too small for vb grenades. I'm going to go with a rifle / mortar hybrid. It looks to be smaller ID than 2 fingers but bigger than one , so between 37mm and 25mm. The Germanicon rifle grenade is 30mm , and this looks about the same. The support has a turnbuckle made in , but no sights , levels , etc. , so one would suppose that it was a relatively short range weapon . There was prob'ly a hinge at the bottom and 1/2 to 1/4 turns on the turnbuckle would make for some fine adjustments. The use of a grenade launching cartridge would also support short range. A sand bag or board weighted with sandbags ( or a couple of kneeling GIs ) would make it stable . Light , fast , handy , able to unleash a flurry of small bombs into a trench line , MG nest , or behind a wall , building , or boulder . That's my take , anyway. Became a dead end with the development of the 60mm and 81mm mortars with thier greater range and payloads and the spigot rifle grenades with prob'ly the same abilities in a smaller package.
Chris