This is a hook eye made to insert into an empty socket.
The inner diameter is 10mm from hex face to face.
I personally made it so I could hang a carabiner from my HSW. Specifically the one shown in the image: https://www.printables.com/model/207725-carabiner It could be used for any number of hook style attachments as well though.
Prints with a little bit of support material on the bottom in the orientation shown. I think you could stand it up on the back of the peg and have it bridge the far end of the ring as well, but have not experimented with that.
The author marked this model as their own original creation.