Hey Henry,
You are correct, this specific mount is designed around the Picatinny rail, and does not fit other propriatary rail systems. Sorry if that was not clear enough.
We do offer free return shipping though, so you won't be out any money. I would recommend considering our M&P 9/40 standard mount, or our Universal 9mm Pistol mount. Both of those should fit your M&P perfectly.
We will email you directly to work through this exchange. Thanks for supporting our small business!
Ben