There also was sight screw hole spacing change on K-frame guns. I once bought a sight stated to fit a Model 66 but found the difference when I tried installing it.
It's been quite a few years so I no longer recall when that change took place but I was trying to install the sight on a 66-2. As I recall, the change took place early in Model 15 or 19 production. Something runs in my mind about the FBI determining while evaluating those guns that the screw hole placement was right over the barrel-cylinder gap and thus made the top strap potentially weak, so the hole was moved front a little. I think the
SCoSW mentions that but I'm too lazy to dig my copy out to verify it.
Ed