Tom,
Yes, that is it, until 1960 (the 1960 Truck Data Book show RPO 223 as the truck heavy duty clutch option and RPO 227 was the car 235 heavy duty clutch option in 1960).
My comment about 1959 RPO-227 (Heavy Duty Clutch) was related to the
JC suffix in the engine serial number of CenCal53's engine.
It looks like JC was used on 4400 trucks in 1959 and on 3100, 3200, 3600, and 3800 trucks with RPO-227 heavy duty clutch.
