Panther almost has it. I believe the split for the c/k designation on the trucks was '71 (my neighbor has a beautiful '70 C10 4x4 with factory big block), it definitely existed in '72... But then the K5 has always been an oddity as it was a "k" when it debuted in '69 regardless of 2/4wd and remaining a k5 through the end of production as we know it in '91... ever seen a C5? Nope. Even the 2wd's (only available from '69-'82) were K5. If we want to further nitpick, starting in '87 when TBI was introduced, all the trucks, blazers, and suburbans (which were still square bodies) began using a "V" designation instead of K to differentiate them from the incoming k-series of '88, since square bodied blazers, suburbans, and 1 ton 4x4's would stick around until '91 (I also had an '88 V30 CC SRW 454/400/205/Gear Vendors) For example, my '87 K5 is technically called a "V10 Blazer" as far as Chevrolet is concerned, yet my '89 Suburban was also called a "V10 Suburban" talk about confusing for the new kid at the parts counter. The new Proline '73 GMC dually body says k3500 on it, when it should actually be called a K35, GMC's just added the 5 (15/25/35, I have a '78 K25 too) and it stuck on newer models of both Chevy and GMC (1500/2500/3500) All said, GM nomenclature is much like the English language; for every rule, there are always exceptions!
