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What makes you say that? Do you know what it's from?
Webs front & rear, no water bypass, longer exhaust snout. If you remove it from the motor, you should find different water tubes arrangement. Probably 1967 or 1968
EDIT: Upon checking, I may be wrong on the webs. Not sure now when they first appeared. But anyhoo, the other items were changed 1967. Remove and check the water tubes for a definitive answer.
How does the grease look? Whatever you decide, I have a good used one here. Can you rebuild it yourself?
No really special tools are required but once you get it dismantled you can push the shift shaft o ring bushing out with a rod that is slightly larger than the shift shaft itself. On your lower unit I think that bushing will push out from top to bottom. That o-ring is often the problem with water getting in to the lower unit. If you decide to replace that housing I have some here that are already dismantled. I could even install a new o ring for you.
Where in Northern WI ??