The early B18 flywheels had a smaller diameter central recess than the later ones. This extra diameter is via small step machined around the recess. This change may have coincided with the use of diaphragm clutches rather than the arm and spring type in about 64-65. I think the central part of the drive plate has a fatter central casting. The later flywheels also have 3 location pins for alignment adjacent to the fixing bolts. Early ones can have the centre machined down and also have pins fitted but re balancing is then advised.
If I knew how to post a pic I would, but will send if you post a usable email address.