US Dragoon & M1840 white buff leather sword knot. Circa 1830s - Mexican War. Plus it still has both braided leather keepers. I'm not sure if they still used this vintage sword knot in the Civil War - would like to know if they did (or did not).
I found one reference that it was used by the Washington Artillery in 1861. Perhaps they dropped it as they standardized their uniforms as the Civil War progressed. Scroll down at http://www.washingtonartillery.com/Uniforms page.htm