This is a drop in Auto Sear for the open bolt Mac 10. But it will also work for the Mac 11/12 variants. I noticed the Full Auto conversion books for the Mac 10 require someone to file down this or cut/grind off that, thus destroying the weapon and its value. No more open bolt weapons were produced after 1982 so it is a collector item. This drop in Auto Sear does not need no modifications to the inside and it will function, firing 1200rpm. Then the AS piece can be removed for later use and no damage to the weapon or its value.