This is the Promag FN 509 9mm 20-round extended magazine.

Features a heat-treated body construction, steel wire spring, and anti-tilt polymer follower. 

This magazine fits the following FN 509-series pistols:

• FN 509 full-size
• FN 509 Tactical full-size
• FN 509 Compact
• FN 509 Compact Tactical

It does not fit the following:

• FN 509 Mid-size
• FN 509 LS Edge
• FN 509 CC Edge