Around 1975 seems to be the changing point from a SS rear site to a black SS rear site,
IIRC late model 66's and all Model 66-1 and 66-2's have a blackened SS rear site that can be identified by the letter "S" stamped underneath,
Many older 66's with SS rear sites were retro fit with the new style black SS site when complaints arose that the older style was difficult to see.
A K target site from a dash 2 or older frame will not fit.
A rear site from a 14-3 ,15-3, 16-3, 17-3, 18-3 and 19-3 up to dash 5 will work on your gun as will the 67 and 67-1 rear site,
A new rounded style appears in the 90's and will not work.