Neither the US 57mm or 75mm RR were officially vehicle mounted at the time. Neither were the German 75mm or 105mm RR or the 88mm 'pupchen'. Sorry to poke holes into your suggestion. We cannot have ahistorical deplyment of equpiment in AH.
(vis the WW, you know how many were ACTUALLY made) Nope, can't allow anything that would deviate from reality in the GV world into the game.