I've never heard that GMC used any "C" designations at all, but I haven't found any factory material for the 67-72 GMC's

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics