Alderan33 wrote:
> Any guesses why a Federal Navy Cuff would have been dropped on the ground near the site of an old bridge
> that was used by pioneers on the Oregon Trail? There was, however, a Union Fort about 5 miles away. (Ft. Caspar)
http://en.wikipedia.org/wiki/Fort_Caspar
The Wikipedia article speaks of the "original" bridge on the Oregon Trail being "several miles" from the newer bridge at what became Fort Caspar.
As mentioned, your US Navy button (with its very-prominent rope-border rim) dates from 1840-1852. We know from civil war reports, and battlefiled digs, that "volunteers" sometimes showed up wearing their dad's (or even grand-dad's) equipment. It's worth mentioning that we civil war relic diggers have found a puzzling large quantity of US Navy buttons at far-inland battlesites ...particularly, in my own relic-digging experience, at the battle of Chancellorsville.
A related possibility regarding your button's location:
As digger-finds in civil war sites show, sometimes when a soldier needed a replacement for a lost button, he wasn't very particular about what the replacement looked like, if it would serve for the need. That is a typical characteristic of Human behavior. ;-)