The only true way to ever know what tranny you have is by opening up the case and counting the gears.

The YS1 trannies were from the 92-93 LS/GS/RS and GSR's only.
The 90 LS/GS and RS trannies were A1's and the 91 trannies were S1's. As far as I know (through the Helms and visual count) there is no differances between the two.

Some tranny codes will not tell you about the gear stack inside or the trim level of the car it came out of. (ie. YS1, Y80, S80)