Yeah, "CK5" is shorthand for Colorado K5, it was never used by GM. There was C10, C20, C30, K5, K10, K20, K30 for Chevy, then C15, C25, C35, K5, K15, K25, K35 for GMC. C denoted 2WD, K denoted 4WD. There is more with the oddball R/V stuff at the end up the square body run and then the subtle changes once the GMT400's were intro'd.