Taking Mr Peabody's way back machine.....

700a is an ETRTO 30-640
700b is an ETRTO 33-635
700c is an ETRTO 39-622
700d is an ETRTO 56-587

650a is an ETRTO 30-590
650b is an ETRTO 33-584
650c is an ETRTO 39-571
650d is an ETRTO 56-537

The more modern designations do not jive with the original intent of the sizing convention, which was millimeters for overall diameter and a letter designation for width. It is a method that is logical, but does not make sense giving the limitations of manufacturing.

How many people claim to be riding 700c that actually have c (39mm) width tires mounted?