Sorry to say, but I've never heard of a C-15 either. I've got 2 of the 67-72 series and I've helped friends with a number of others. All C10s are considered 1/2 ton capacity trucks. The C20s are 3/4 ton and the C30s are 1 ton. From '68 on there was a XL bed option but it wasn't always called "Longhorn," which I believe started in 69 or 70 is based on a C20 or C30 chassis and ment to haul a big camper. My '71 C10 is a 7' fleetside bed, but there are 7' stepsides, too. There would be C15s all over the place if it were a long bed designation, since most of the smaller trucks from this period are long bed! Your truck is a C10 longbed. It could have any number of trim options. If it has six bolt wheels, its a C10.
btw, look at the sticker on the inside of the glovebox door for accessories, etc. on YOUR truck.
best wishes,
Les
