Used Motor parts from a 1976 Harley-Davidson Sportster. Bought this bike reposed from auction as a girl had beat it up with an ax. True story, she must have been pretty mad. Both cylinder and head have fins broken off from an ax shot, see photos. I believe these are weldable if you know what you're doing, maybe salvage fins from a junk jug. I think it is the rear cylinder, the cast numbers in photos will tell you for sure.