A die-cast transmission extension housing has an outer peripheral wall or whell and an integral bearing supporting partition wall provided transversely of the longitudinal axis of the shell between the open forward end and the closed rear end thereof. The partition wall supports at least a bearing for the transmission output shaft extending through the open forward end, the bearing on the partition wall and the closed end. The inner surfaces of the shell forwardly of the rear surface of the partition diverge to the open end of the shell so that the inner space defined by these inner surfaces and the partition can be moulded by a die-casting mould part which is slidable out of the inner space through the open end of the shell after the extension housing is die-cast. The upper wall of the extension housing is formed therein with an upper opening adapted to be closed by a shift lever retainer. The side and bottom walls of the shell cooperate with the partition wall to define a second inner space which is so shaped as to be moulded by another die-casting mould part which is slidable out of the second inner space through the upper opening after the extension housing is die-cast.