Having worked at GM over the years I've seen many revisions with fasteners used on cars and parts. It may have been a brilliant idea for a job setter on a line to come up with using the posi drives. They could have used both phillips and posi drive bolts at different times during production. You don't see a lot of these type of bolts in that time period at GM. They were a special thing just to have clearance for the hood. We didn't have these in the bolt bins near the parts cribs. Not a standard fastener in the US auto industry.... These days there would have been a socket button head used.
