The posters below are correct, Durkee.
Ns were originallyequipped with the then standard 1 1/8" PTO shaft and many many PTO pulleys for 1 1/8" shafts were made and sold.
Most Ns have subsequently been retrofitted with the now standard 1 3/8" PTO shaft.
By the time that the 1 3/8" shaft became common, few folks were still using belt driven equipment so few PTO pulleys for 1 3/8" PTO shafts were sold. As a result, these PTO pulleys are much less common.