This is the ProMag FN 509 9mm 10-round magazine.

Save some of your hard-earned money without sacrificing reliability with this 10-round FN 509 magazine from ProMag!

Constructed from heat-treated steel and featuring an impact-resistant polymer floorplate, you will never have to worry about durability. Inside, you’ll find an injection-molded polymer follower and a chrome silicon wire spring that provide the reliability to match. 

Your FN 509 is versatile enough to handle almost any task, so you can never have too many magazines ready to go!  

At this price, you might as well order a few!