FN 20100348 509 15rd 9mm Luger FN 509 Midsize Black Stainless Steel
https://www.tandktactical.com/shop/845737010126-fn-20100348-509-15rd-9mm-luger-fn-509-midsize-black-stainless-steel-17470
https://www.tandktactical.com/web/image/product.template/17470/image_1920?unique=a77d9a6
This magazine is compatible with your FN 509 Midsize.