Per the Standard Catalog of S&W:
Product code 100563, six shot four inch square butt, no underlug, 200 manufactured in 1991.
Product code 100564 six shot 4 inch square butt, underlug, manufactured 1990-1995.
Product code 100565 six shot 4 inch round butt with square butt grips, underlug, manufactured 1995-2001.
All the other listings are for 10 shot or special run round butts, so yes, it was only made for a few years as a six shot square butt gun.