Factory (per 66 Engine Assy Manual, AM0156 pg36) - C4TE-8620-AC, 40.25 X 15/16.
Service (per Ford Car Parts, sec 86.1 pgs 6, 20 & 21) -C9PZ-8620-S, 41 1/4 X 15/32, JB-412.
I think the JB-412 belts were marked with the factory number (C4TE-8620-AC) and are the current selling item. My 65 does have a belt marked C4TE-8620-S but I put that one on at least 15 years ago. Factory is same as 66 (AM155 pg12).
Jim