Rear Seals for 904

Morning FABO,

I have been surfing the forums lately and have found a couple threads with comments that say there a couple of different sized rear 904 seals (or tail shaft seals). Does anyone know how I am able to identify which seal I have and/need? My dusters a G code car but has been magnum swapped and the 904 has supposedly been rebuilt, I do not know if its the original transmission. Thank you for looking, any input and advice would be appreciated.

Ethan