Standard Catalog of S&W states that the Model 29 was changed to a round butt in 1995-1996. Model 610 was always round butt, Model 625 was always round butt.
Model 624 was square butt, Lew Horton specials were round butt around 1985. Model 27 was square butt, discontinued 1994. Model 627 was always round butt. Model 28 was always square butt, discontinued 1986. Model 25 was square butt, discontinued 1999, reintroduced in 2001, may be round butt now? There was a fixed sight round butt variant around 1989. Model 24-3 was reintroduced in 1983-1984 on the square butt, Lew Horton specials on the round butt.
Hope this helps.