For "normal" production Six Series double actions. The Speed Six is round butt. The Security Six and POLICE Service Six are square butt.
However, rare, uncommon and odd variations have happened.
For example, there are a few factory ROUND butt POLICE Service Six's. I own one. As I understand it, they were built for a law enforcement contract that either wasn't completed or a few guns extra were made.
Tell us more please.
What do you have and/or why do you ask?
Oh, and this has been discussed in other threads too if you want to poke around using the Search feature.