4" and 6" model 19's and 66's were only available in square butt until S&W discontinued the square butt in the mid 90's.

Any round butt 4" or 6" 19's or 66's are either conversions or some special order. I believe some government agency bought a small order of round butt 4" model 19's at some point.

Also, all of the 3" model 19's or 66's I have seen were round butt. I would feel if you found a square butt 19 or 66, it likely was converted.