My guess is it changed when they ran out of old bolts. When I restored a 65 vette it had the julian dated shocks on it with bolts that were wrong by ncrs records. Come on nobody changes all the bolts on the original shocks they where left over bolts from 64. Also had 2 different head marked bolts that could be used for bumpers right side had one left side had the other. It was whatever barrel they rolled out of stock when the ran out.

Mike