I'm sure there are members here who know the answer to this without looking it up, at least I knew where to find the answer.

No. Typically round butts were on guns with barrels shorter than 4" until 1995-96 when S&W went to the "all round butt" way of thinking. The 66-4 appears to have been made from 1994 to 1998 so, while there should be some square butt 66-4s, they won't all be regardless of barrel length.

Back to the PC magna grips. I dug out a couple of handguns from my safe and found my 686 was wearing a set of PCs. What I didn't notice from the earlier pictures posted was just how diminutive the checkered area was on them as compared to a standard K frame magna grip.