The clutch head screws were actually used at least through the 1970's, maybe later. When I worked at a Chevrolet dealer in the late 70's-early 80's I had a late 70's Chevrolet custom van that I had to replace a quarter panel on. The rear floor was carpeted & stapled to plywood,and the plywood was screwed to the van floor with clutch head screws. I had no idea what they were and one of the older bodyman in the shop had a set of clutch head drivers he let me use.
