Hi TLea,
According the Ford Master Parts Catalog, it is only the 1965-66 F100's and F250's with the straight 300 CID that received the "C" servo. The same years trucks with the 240 CID received the "A" servo. The engineering part number shown shows its origin was Galaxie C5AZ-7D027-B with the "A" in the prefix part number. If you can find a police or taxi Galaxie from 1965 or 1966 with the C4, it too should have the "C" servo for heavy duty.
You are right, these are also very hard to find.
Take care
Cory
P.S. Don't confuse casting part number with engineering part number. They are usually different.