The Rebel is correct. More specifically, your brass "gag loop swivel" is a piece of horse-harness from the bridle-&-reins assembly. An exact match for yours is found in late-1800s/early-1900s horseharness catalogs, and yours most probably dates from that time... but I should also mention, they're still being made and sold today. See the 1907 mail-order catalog scans, and the modern-made version in the photo below -- the gag loop swivel is the component on the left in the photo of the "gag chain."