No doubt it was mounted to leather, due to the very small distance from the back of the "4" to the inside of the rivet head. Very well could be off of civil war era horse tack. Maybe horses that pulled caissons, cannons, or other equipment that had to have numbered sets for inventory and ease of assembly? That font style is antique too. Sub
I dug a "1" almost identical to yours in a city park a few weeks ago. Mine turned out to be a house number. Mine was also 19th century and made of solid brass.
Well, I thought about house numbers too initially. But I'm so far in the country they didn't have them till recently and more importantly, this thing has rivets like to attach to leather.