The "C" originally represented a 1960 series Ford part number. The "C-4" got that designation because it was first introduced in 1964 (hence C for 60s and 4 for 64) for small blocks to replace the Fordomatics and started with a part number of C4AZ or C4OZ, depending on application. The CruiseOmatic was large and heavy so was never considered for the small block, but came back in modified form as the FMX for use with the 351/400 engines in the '70's. The CruiseOmatic was replaced with the "C-6" because it was introduced in 1966. The only thing that throws a wrench into all this is the "C-3" and "C-5". The "C-3" came out in the '70's as a cheaper, lighter trans for use with low powered 4 & 6 cylinder engines (which in the mid-late '70's was just about all of them!!!). I have been told that it originated from a European Borg-Warner, but I don't know that to be fact. The "C-5" was introduced in 1982 and was phased out in 1986 and replaced by the AOD. The "C-5" is a modified "C-4" with a lockup torque convertor.