This was posted by Cannonball guy . the man knows his relics.
As an addition to the diagram helpfully posted by Old Pueblo, here's a modern photo showing rein-guides, which are the rings on the thick black leather strap/harness in the photo. They are for wagon/coach/buggy-pulling horses' harness. The terrets/rein-guides keep the long reins from drooping down along the horse's sides, where the reins could snag on something. The "swinger" terrets, like you found, are a fancier version of the simple ring terrets/rein-guides.
The other photo shows simple rings and another variety in the middle, in which the ring has an opening, for ease of inserting the "bearing rein" seen in Old Pueblo's diagram.
Goodyguy asked: > Never saw one with a lid before. Wonder what the lid is for?
That is a version of horseharness reins-guide technically called a terret) called a "swinger terret." See the circa-1900 horsegear catalog advertisement below. Cclantz's find appears to be the one at far right in the second row.
OK, I have a question, Were these functional or simply decorative? I don't see how they could function because the reins would wedge in them and the only time you could see the "fancy" was when there were no reins in them.
OK, I have a question, Were these functional or simply decorative? I don't see how they could function because the reins would wedge in them and the only time you could see the "fancy" was when there were no reins in them.
Ticndig found a relic-ID photo I posted as helpful, so here's another. It shows that reinguides/terrets are still made and being used today, seen here on a buggy's harness "in action."