Elastic covers for your swivels. They are especially useful to protect the support hand when using the forward swivel as a hand-stop. Other benefits include keeping the swivels silent and tidying the look of the sling. They can also mitigate push button swivels turning when used in flush cups that aren't rotation limiting, as the elastic adds some friction against the stock.
A must have in my opinion if using MASH hooks. Extremely beneficial on all swivels that attach to studs. Probably unnecessary on push button swivels, except to limit rotation if needed.
Made for 1.25" webbing only.