I'm working from some notes I made just after Moses parted the Red Sea so bear with me.
The 65 condenser that attaches to the alternator has a center mounting bracket/tab and is hard to find.
The 66 and on that attaches to the regulator has an end mounting bracket/tab. That's the physical difference, maybe. P/n and markings are all on the end: 65 type - C5TA-18832-A, 648 in an oval , not sure about other markings; 66 type - C3SA-18832-A, 648 in an oval, no other marking; C6OA-18832-A, 648 in an oval and "FOMOCO" in an oval, or "AUTOLITE" in several different fonts and type sizes preceeded by an Autolite logo (a circle with spikes at 9, 12 and 3 o'clock). The 648 is the supplier's model number. Simple, no?
Jim
I found another p/n for the 65 application, C5AZ-18827-A, 913 on the end and FOMOCO in an oval. This is for the center mount type.
The service replacements for 66 on are C6OZ-18832-A and D5AZ-18832-A as of 5-75. Those last two p/n's will be the box marking, there's no telling what will be marked on the part.