I have a 64 C10 truck, owned it since 1996, and I would have sworn it was all original. I would also have sworn all 63-66 C10 rear axle assemblies were the same. I found out recently while replacing axles that the splines changed from 17 in 1964 to 30 for 65-66, and that the location on the housing to mount the panhard bar moved three times over the years. The 30 spline axles and panhard bar location proved that somewhere over the years, a 65 third member had been swapped into my truck. The dead giveaway was that the panhard bar on my truck had been heated and bent slightly to make it shorter so it would fit the 65 housing. No one was concerned about maintaining originality back in the day.