This magazine is compatible with your FN 509 Compact. It features a stainless steel body with a flat black finish, high visibility anti-tilt follower, and extended floorplate. *Note- This mag will not work with FNS or FNX pistols.
Specifications:

Caliber	9mm Luger
Capacity	12rd
Finish	Black
Model	509
Material	Steel
Type
Model Fit	509 Compact
Brand Fit	FN
Capacity	12