I have a Kodak Colour Dataguide from 1968 and another from 1974 which both came with a negative [Ektacolor Professional S - C22 Process] and an accompanying 2x3 inch print. They were definitely part and parcel of the original publication as the envelope's edge is incorporated into the spiral binding.

I really wish I'd been in the right place at the right time to be on the receiving end of some of those prints in the French magazines that Ian mentioned.