My 52 GMC came with the same type "c" clips as the Chevy. Here is a picture of an original upholstery 49 GMC to give you an idea of the fabric.
49 seat back Notice the 49 seat bottom has a metal band around it while my 52 does not.
seat bottom SMS in Portland makes a good looking reproduction of the fabric. The thing that is different between a Chevy and GMC is the type seam stitching. GMC used French seam while Chevy used the bead.
