This is a factory FN 509C 9mm 17 round extended magazine. 

This magazine is the FN 509 17 round magazine with a custom sleeve, allowing it to fit flush in your FN 509C pistol.

Features:

• Heat-treated stainless steel body construction
• Anti-corrosive steel wire spring
• High-visibility orange anti-tilt polymer follower for reliable feeding
• Rear-facing, numbered round indicators for quick round counts
• Rugged grip on sleeve for secure grip