My guess is the previous owner either swapped the whole gear case with one from a 1958 or later 18 HP because it’s an even swap except the prop shaft bearing head has the smaller I.D. like you found. Or just swapped out the head because the original was scored, but swapped it with the ’58 and later version. Or it could be as Mike mentions with the serial numbers. The parts page shows a mid year change for the seal. See #35
The larger seal is 302495, the smaller one is 302545 > 313284. Ebay has a bunch of them for $12-$15.
