The Security-Six and the Service-Six both came with the square butt. Only the Speed-Six, in .357 and 9mm, came in round-butt. However, any square butt can be easily changed to a round butt with an electric grinder.
Back in the early 80s, a technical fellow at the NRA showed me his converted Security-Six revolver. He'd written an article on the .357 for the
American Rifleman in which he reviewed the best selling magnums of the day and it was clear that he favored the Ruger Security-Six by far over the rest. He'd shot a number of K-frame magnums to the point they were no longer able to be retimed (because the frames had warped out of shape), but the Ruger's solid frame made it all but impervious to full house magnum rounds.
But what was really nice, the Ruger, like the K-frame Smith, could have it's grip bobbed for not only great comfort, but greater concealability.
Here are two of my boys, both with originally square butts, but converted to round butts. If you think they're nice looking, you should try holding them. You can't get this kind of configuration out of today's Rugers since all you get, really, is a post, around which your grips must be configured.