FN 20100262 503 6rd 9mm Luger Stainless/Black
http://www.tandktactical.com/shop/845737009106-fn-20100262-503-6rd-9mm-luger-stainless-black-23521
http://www.tandktactical.com/web/image/product.template/23521/image_1920?unique=cbc5fa3
This magazine is compatible with your FN 503. It features a stainless steel body with a black base pad.
