Williams makes a great receiver sight for older Marlins with the 2 little holes on the left side of the receiver, and a new one that utilizes the aft 2 scope base holes on the newer Marlins. I have used both with no problems. You can even get a version with knobs so you don't need a screwdriver. Lyman makes a fine receiver sight for the older Marlins, but I don't know if they have a version for the newer ones. The Lyman 66 is made of steel, if I remember correctly. The Williams sights are alloy. Good luck!