Trannys went from loose needle bearing to a caged roller bearing for the high speed gear and the countersahft.

When you pull the transmission out and remove the countershaft you will readily be able to tell.

 

Panzer is correct that was the main change was the transition from loose rollers to needle bearings. I would guess that the S code is for a 76 even though the S and O were left off the list. The O was probably omitted so as not to be mistaken for a 0 (zero)