Joe:
RPO is GM abbreviation for Regular Production Option
The block casting # 3701481 is for a Chevrolet 235.5 cu Passenger Car with Powerglide Transmission.
The cylinder head #3701887 matches all stock 1953 235.5cu engines for that car.
That engine used the SAE (Society of Automotive Engineers) nomenclatured "Full-Pressure" lubrication system.
