Just for what it's worth, which might not be much, there also were two different length K-frame sights with the older style square-ended tang. Here's a shot of a Model 66-2 that came with the "newer old-style" sight and an "older old-style" beside it.
I found that out when I bought that extra sight online and tried to install it on that gun. The seller wasn't aware of the different lengths either and cheerfully accepted the sight as a return.
Research (the
SCoSW?) taught me that when the FBI(?) evaluated sample K-frame revolvers sent to them prior to S&W making the guns in their order, it was thought that the hole for the older sight being right over the barrel-cylinder gap could be problematic so it was moved forward a tad.
Ed