This is the ProMag Ruger SR9 9mm 17-round magazine. 

Feed your Ruger SR9 with this reliable, high-quality 17-round magazine from ProMag!

The heat-treated steel construction is finished with a black oxide coating, which provides superior corrosion resistance and toughness. Internally, you’ll find a chrome silicon wire spring and injection-molded polymer follower that feed your pistol flawlessly.

Side-facing witness holes allow you to keep track of your remaining rounds, and the baseplate is removable for easy maintenance. There is nothing left to be desired. A lifetime warranty backs each magazine, so you can rest easy! 

Save a few bucks by picking up a few of these ProMag 17-round magazines over the factory ones! You won’t regret it!