I picked up a 681 a couple of weeks ago and contacted S&W about no "M" via email. I included the serial number. They sent me a reply back saying my gun was safe to shoot magnums.
Been a .40 advocate since the early 90's when our department issued the Glock 22. I LOVE it! But the first time I fired a .357Sig I said I have got to get me one of these. It is a nice option to have both. I will not give up either one. But if I could only choose one, it would have to be the .40...