I agree with BigCypressHunter ...specifically, it is a horse-harness rein guide (also called a terret) with a swinger. I've seen some other "dug" swinger-terrets which look just like the one in Tater307's photo, except that the stud-mount's base isn't broken off like his specimen's is.
By the way... it is not silver-plated. That is "Nickel-silver" plating (which actually contains no true Silver at all). Real Silver plating is almost never used on objects intended for continuous outdoor use, because Silver develops a dark tarnish pretty quickly when left out in the weather.
Nickel-silver has a Lead-ish look to it in comparison to Silver, which by comparison is white.