I just replaced the rear pto seal (I assume this is the seal you are talking about and not the rear main seal) on my model C. It took Chicago Rawhide part number 11800.
According to my parts manual, there are two different diameter pto shafts on the B and C's, 1 1/8" and 1 3/8". Mine has the 1 1/8" shaft.
Good luck!