The 91A shift fork is used with the '39 and up gears.It was put in 39 and later top shift trans. The correct case would be a 78- case. It was used thru 48 in the pick ups. I have found the gears in a lot of cases that they did not start out in. It has always been a good swap. The gears can be identified by the two toothed rings on each side of the shift sleeve. The early shift system only had one. The shift sleeve has the fork groove all the way towards the rear on the 39 up gear sets.