This is the XTECH MTX S&W M&P Shield 9MM +2 / .40 S&W +1 magazine extension. 

Modifying your magazine needs to be done correctly, or your handgun can turn into a giant paperweight. With these patented magazine extensions from XTECH, you can quickly and easily extend your capacity without the need for a single tool!

With the custom-engineered spring that is included, you can rest easy knowing your OEM magazine will remain reliable, all while adding an extra round or two. Its polymer base pad was designed to provide an OEM look, but it serves as much more than just a fancy-looking accessory. 

These extensions are a no-brainer, as they extend your capacity and improve your control via the extra surface area for your grip.

Grab an extension for your carry mag, range mag, and home defense mag! The extra rounds can add up!