Used Vortex Viper pistol red dot. Pretty heavy use, has carry/holster wear, it works fine but Vortex also has a tremendous lifetime warranty if you didn't already know. 
comes with FN 509 mounting hardware, including the inexplicable little O ring. 

I also have a level 2 owb holder for a 509 with a streamlight tlr7 in another listing, will make a deal for both.