The letter+number is the Kawasaki 'short form' notation indicating the model and year of a machine.
Kawasaki C1 is a 1990 ZR750; C2 is a 1991 ZR750, etc.
Some folks use the short form to indicate what they have, others use the long form. As long as the seller is accurate in his representation, either designation is valid. To be helpful, Gary provided both designations in his article about the 17" wheel upgrade, the source of your wheel list.
FYI, a 1986 Connie ZG1000 is A1, your '06 is A21.
Also FYI, as far as I know the terms C-10 and C-14 in reference to 1st and 2nd generation Connies are not official notations but are only popularized by Connie Fan Clubs (maybe someone will correct my understanding about that)
HTH
