It's the single point sling mount, you have to remove the stock, it goes between the stock and the lower.

The thing is I don't know how to remove the stock.

HELP!!!

get the tele stock tool  and unscrew the castle nut and remove the old rear plate and put the nwe one in its place. When you take the old one off make sure you do not lose the spring and detent . and reinstal it just like it was. with the buffer tube just over the buffer detent

this is what you need to take it off

i48.photobucket.com/albums/f248/theinvisiblesoul/223-telewrench2.jpg

This is how the tube should look when done

i48.photobucket.com/albums/f248/theinvisiblesoul/MyARbuffer.jpg

                              Invisiblesoul

try this www.jtrusty.com/?page=article/read&id=12
or this www.ar15.com/content/guides/assembly/lower/

Great help, especially the images, thanks!!!

did you figure it out ?

             Invisiblesoul