Here's a possibility, and a guess only. I looked up patent dates of July 15, 1869 and got several hits from Scientific American which list weekly patents issued in the summer of 1869. There's several items listed for your date, including a "thill coupling", or the portion of a carriage or wagon which attaches to the animal. Here's the patent link (http://www.scientificamerican.com/article/official-list-of-patents-1869-08-07/). Do a "Control + f and search for "15" and you'll see a few hits. They make drive couplings today on motors, so something like a washer from and old thill coupling could be it. Here's another look at one - http://4.bp.blogspot.com/-PKIUbHdkvi0/UuqhhAQnwNI/AAAAAAAAH1E/hAElqaVoPu0/s1600/Horse+Detacher.jpg.