277594 Crankshaft & sleeve assy. was replaced by 204100. Likewise, 203711, 203657 were also replaced by 204100. So 204100 is the latest number. I THINK 204100 had a chrome plated end where the seal rides. As you suspect, the 1957-18hp carbon seal crank won’t fit in he 15hp case because of the lower bearing/bushing. All of the above mentioned cranks used the troublesome 303804 lip-type seal. I think there might be an improved version of the 303804 also. More research required to find out.
EDIT: 303804 seal is superseded by 0321467. I have one here in front of me. It looks like better quality.
