Found my old Haynes book and according to it there were 3 kinds . The fixed, floating and the sliding calipers and I'm guessing that mine are the sliding type. Thanks for an education.
Correct! From the pictures yours are the sliding type. The fixed ones are the earlier SBP Kelsey Hayes 4 piston brakes. The floating ones are the pin-style calipers, they started using them in '70 on the B/E bodies. Internally they're basically the same single piston caliper, they just use the pin style caliper brackets. And the sliders are the one's that started in '73 on the A-bodies. Not sure if the sliders started before '73 on a different platform or not, I know the B/E bodies were pin style before that. Pretty much all the other body styles used the 2.75" diameter piston regardless of whether they were sliding or floating (pin style) calipers. Only the 73-76 A-bodies use the 2.6" piston calipers, and I believe those are all sliding calipers. After that they were all 2.75". Except the C-bodies, but they were pretty much their own thing the whole time, some years were the same as the 1/2 ton trucks.