It appears he was transferred FROM 3429th Ordnance TO Company C, 423d Infantry - no doubt one of those transferred in to rebuild the regiment AFTER the action at Schonburg. Primary role of an ordnance maintainance company was the handling of ammunition, etc. He'd have been assigned to the motor pool (every unit had a motor pool). The letter from 3429th to C/423d is merely confirmation that he's entitled to a bronze star device to be worn on the ETO ribbon to denote participation in the Rhineland Campaign. This means simply that he served somewhere - probably the motor pool of 3429th Ordnance - within the prescribed area of operations during the allotted time frame. Nothing to do with award of the Bronze Star Medal or even having actually participated in combat.