Depending on the brand of the clamp, the large hole fits an Uncle Mike's quick detachable, push-button sling swivel, OR a stud that takes a standard swivel.
Here's a link to the type of push-button swivel that locks into the clamp hole:
http://www.unclemikes.com/adtemplat...y=1279575&subcatky1=4548842&subcatky2=2000188
To use, you simply press the button, and insert the end into the hole in the clamp.
You don't need to use the lock base if the hole in the clamp is the right size.
Another type, usually used on the Remington factory clamp, is a stud that's riveted into the hole in the clamp.
On this type, a standard quick detachable swivel hooks into the hole in the stud.
Here's the Uncle Mike's version of the Remington factory clamp.
Note the stud riveted into the bottom of the clamp.
http://www.unclemikes.com/adtemplat...y=1465723&subcatky1=4548843&subcatky2=2000180
The swivel just loops through the hole in the stud.
In my opinion, the push-button swivel is neater. You don't have to rivet the stud into the clamp, and with the swivel off, there's no stud sticking out in the way.
