It seems that the GMC trucks of this vintage commonly used Philips head screws for the interior. I thought that clutch head screws were consistent on the Chevys. I have seen some variability in clutch head fasteners on other parts of the trucks, and date of manufacture did not appear to show a trend. Perhaps assembly plants had some autonomy. The ones I have samples from were assembled in Atlanta and Baltimore.
What is unique about these slotted screws? A picture would say a thousand words.
