OK, well 304265 does not supercede, so the safe answer is "no", the 321929 shaft will not work for you. Is this the last word on your question, well no, but you would need to do some homework on just what the differences are…Do you know what year/model uses the 321929?
It is for the later 1960 and up 18/20/25hp into the late 70s. I can let you know later what years exactly, but suffice it to say, not for your 1957. Do you need one? Im sure a local member could help? Pretty common part, if not let me know.
Don’t badly need one. One of my engines I replaced the seal and it still leaks at the seal because of the groove but I will try to position the seal in to where it will be out of the groove and that might work.