I don’t really have the answer, but I do know that 3755-5100 fits up thru 1956 with the water pump up below the powerhead. Does your 1957 have the pump down on the gearcase? Sorry, I don’t have the parts book for that one.
EDIT: From what I’ve been able to dig up (not reliable) it appears that 1957 still had the pump under the powerhead. If that’s true, 3755-5100 is the correct impeller.
