Found this MDing on a friends property in the country. Not sure what metal it is, but I'm guessing bronze. If it is some sort of seal or washer, I've never seen one like it. First pic shows it in the best light, second pic has bad lighting but shows scale.
I am going to say bushing as well. it has the keyway tab on it on the inside, to fit over a slotted axle maybe a shaft. For some reason it reminds me of the ones on my lawnmower except I think yours is slightly bigger.
Not a bushing. Maybe a spacer or thrust washer. If it's made of brass or bronze, probably water related. It wouldn't be for any modern type axle as those spacer/washers are made of steel so they can handle heat, weight, and side thrust.