Hard to argue with a picture.
So, I went and read the section on 66's and sure enough they went to round butts only starting in 1996 just before the MIM started.
So yeah, I was wrong, at least as far as the late model 66's are concerned. In my defense, however feeble, I have no interest in late model S&W's and therefore do not keep track of them.
That said, it appears all 3"er's are round butt.