I think the 6906 can also use the 15 roiund magazine. I understand that there is an adapter to take up the difference between the magazine length and the base of the grip.
Order them from S&W. Don't recall the part number, but that's where I got them. They're called the "+2 Adapter". They'll only work with the 5906 mags, not the 659 mags.
Another 6906 question
Linear Mode
