I found this number on the side of the case: 9153011020R
The number on the rib on the bottom of the case is pretty much wore off. Where else might I found the number?
Also, from the appearance it would seem as though this transmission sat outside for a long time. Note the rust on the flanges, fork and input shaft. Everything turns really smoothly though and the gear selector seems to move nicely. Can this rust be cleaned off such that parts are not damaged (Ie: the splines on the shaft are not destroyed)?