See proficiency ratings here:
https://www.flightglobal.com/FlightP...20-%201328.PDF
C = Administrative Ability.
I believe 652 refers to the section? or paragraph? related to Discharge Headings/Reasons as found in "The King's Regulations and Air Council Instructions for the Royal Air Force" (KR) probably a 1940s edition.
Example:
https://www.forces-war-records.co.uk...eryExact=False