Mec-Gar makes some OEM & gov't contract mags (most notably for Berettas & SIGs), but I've never heard that they make them for S&W. (Not saying it isn't so - just that I've owned S&W autos for years and never heard that.)
I have used Mec-Gar mags in "59-series" S&Ws without incident, but I've never personally used them in any of our "69-series" pistols.
Believe it or not (many won't), the Ram-Line extra-capacity mags (the ones with the "constant-force spring") for the 69-series S&Ws work very reliably in my experience - I have several and have used them quite a bit over the years in a 6946 and a pair of 469s (my wife's). Unfortunately, I believe they have been out of production for some time.
I wouldn't hesitate to try Mec-Gars in your S&W, even if it turns out they aren't OEM suppliers. In my experience, the S&W autos are a little more "forgiving" of aftermarket magazines than many other pistol designs (again, I've had/still have a bunch of them).