[tekuhn]

The reason you see a big visual difference is that the early rods through 1962 were forged and then the switch to cast in 1963 (except for a few high performance engines, of course). Early forged rods were no stronger than the later cast because they still Rockwell test quite soft, but they do tend to fail less catastrophically (stretch vs. break) so we used to run them before the aftermarket finally started producing a reasonably priced forged alternative.