On MarineEngine I checked your model 3 HP and all of them up to 1968 and the only year that shows an o-ring in addition to the slinger is 1966. But when I check the number for that o-ring 308117, it says it’s an invalid part number. There’s also a discussion on MarineEnginge about someone else who has a 3 HP that shows the o-ring but his is a 1968 and when you check that parts page it doesn’t show it. So maybe if you just take it to a hardware store or buy an O-ring assortment kit from Harbor Freight you might be able to match it up.
