I saw that in your other posting on the clay marble and button. Thought, "hmmm, guess truth didn't think much of the rivet because he didn't mention it."
Seeing as how that Fort has such long and varied history and even as an amusement park it could have come from any number of sources, jeans if not too deep, horse tack if fairly deep or any type of material equipment. Maybe even a sheet metal rivet from rides in the amusement park. Hard to say without analyzing. Cool