This is the ProMag Smith & Wesson SD40 .40 S&W 10-round magazine. 

Improve your comfort and control thanks to the floor plate with finger rest on this high-quality 10-round magazine from ProMag!

Designed with durability and reliability in mind, ProMag combines a heat-treated steel body with an impact-resistant polymer follower. These not only accomplish both but also mitigate excess weight. 

If you ever need to perform maintenance on the magazine, the floorplate is quick and easy to remove. Additionally, the side-facing witness holes ensure you are always aware of your remaining rounds. 

What more could you ask for, aside from a few more rounds?!

At only ten rounds a piece, you might as well grab a few!