I recently was given a very old Craftsman 6HP with a 20-21" scoop.
snowblower. It started on one pull and ran perfectly but in testing I
found the auger was slipping.

On further inspection I can see the chain is badly stretched. I could
pull it off easily with just my hands.

I proceeded to pull the cover off and when investigating the chain a
small rectangular pin about an inch long fell out of the
engage/disengage mechanism on the transmission shaft.

I'm guessing it is some kind of shearing pin but for the life of me I
cannot figure out where this pin goes?