C139 should be March 13, 1959. The "848" heads were the casting number for the heads used 56-to 60 at least, cuz I pulled an 848 head off my '60 235 when I rebuilt it. Some 58-62 engines had the side motor mounts even if not used.
Most 55-62 2nd series 235 parts will interchange, except for some starters, valve lifters, pulleys, water pumps... Most of the Full Pressure 235's engineering changes over the years were to the internal oiling system and main bearings and happened between 1954-1957.
