On their first design for the clamp, they used a 12 point internal wrench nut to clamp it down and hold on the sling swivel.
Why I have no idea, finding a tool that fits is next to impossible. All the auto part stores carry the next size up and I cannot even find one online. I went to check ACE's site and I see they now have an improved clamp where the sling and the clamp nut are separate.
I think I will call ACE and buy the updated part if I have to, it seems the old ones were loosening up probably because noone could find the proper tool. Rather spend $20 on the new clamp than $20 on a tool I will never use again.
Plus the new clamp seems to have a true 360 swivel. I hate buying stuff when it first comes and then by when I actually find time to use or assemble it, find there is already an improved part out there.