I held the opinion that the rear derailleur got more attention prior to 1971, the transition of the surface at the top pivot was hand dressed often to an organic surface flow, the later ones were not dressed.
On the front mechanism, Campagnolo did it appears make some process change, material or casting method or post casting finish that leaves the body and clamp on the earlier units (before the extension to beef up the top pivot root)
dull and sometimes rougher.
It was the lack of a cir-clip on these intermediate pieces with no screw retention that caused more than a few to bend the upper pivot, I had to replace the body on my 1971-72 production front to fix this. It caused slow downshifts as the pivots were no longer parallel.