All of the PRESENTLY-KNOWN evidence points to the various horseharness loop-shaped "swivels" as being from sometime after the end of the civil war. Or at least, not from Military-issue equipment.
The evidence:
There are lots of photos showing a civil war soldier on horseback, and none of those presently-known photos show the various horseharness loop-shaped "swivels." If somebody here knows of such a photo, from 1861-65 (or earlier), please post it.
Also, no civil war era horsegear seller's catalog has surfaced which shows (or lists) the loop-shaped "swivels."
The situation is like what killed the belief that brass tent-rope adjusters (also called tent-rope "slips") are from the civil war. There are many photos showing civil war soldiers at their tents. Not a single one of those many photos showed the brass tent-rope adjusters. Eventually, on-paper Historical Documents were found which proved they are from no earlier than the 1880s. (See the inventor's US Patent diagram below, dated Nov. 30, 1880.)
Here's an example of a photo of Military-issue horseharness on the horse. Note that there is no version of the brass loop-shaped "swivels" on the horsegear.
But as I said above, I am speaking about the presently-known evidence. If somebody here has solid proof of 1861-65 usage of the loop-shaped "swivels," please post it.