Shadetree, The carburetor in 51 Merc's picture appears to be tilted backward. Would this mean that it more likely was a boat engine than a plane engine?
They appear to be Fenton, see the casting just under the radiator hose.
What radiator hose. Boat or airplane both are/were air cooled. JMO
Here in New Zealand there were twin plug heads made in the 50s or 60s by Ron Hogan, of which some survive today. I believe his son Garth may still have the patterns and may have produced some.I believe it was KONG that offered a two plug per cyc. head, early 50's...and NASH had a 16 plug wire dist, and didn't Bubbna show use a 16 plug wire dist, a short wile back, and next to impossible to find parts for. ??? ....OLD....BILL