Calipers are made in different versions in particular years,so the caliper bore and piston size may be the same but the style of boot different and so the rebuild kits may have the other option included. Also back then they usually used the phenol resin("plastic") piston as opposed to the steel piston. The phenol ones would wear and sometimes stick in the bore overheating the pads in the process.Steel pistons are better in my opinion,but only if the car doesn"t sit around a lot we used to rebuild a lot of those back in the day......................