The Feee Master
Jr. Member
View attachment 1500496View attachment 1500497View attachment 1500498 3/8 inches long, the smaller circle is 3/8 inches in diameter.
It is a version of copper/brass rivet most commonly used to connect two (or more) thick pieces of leather for horse-harnesses and saddlery. Still being made and used today, but yours is an older style, common in the 1800s and early-1900s.
Cannonballguy hit it dead on. Sometimes we find these in Civil War camps.