Need some info. on a 4 speed muncie we are now looking to purchase. It has a U.S. patent #8088336, there are (2) numbers stamped on the case, they are #3885010 and # 6K116565, side cover is 3831707 and the tailshaft housing # 3857584.. Just need to know is it a M-20 or M-21 and the year? Thanks.......