They look like dandy buttons. The CannonBallGuy told me a bit about them last year when I found a few myself. I believe he said the larger ones (~28mm+) are older, 1770-90, and they get progressively smaller leading up to the 1830s. "Dandy" buttons were so named because they were worn by "dandies" - aristocratic-type people who dressed elegantly during their daily lives. They may be brass but also are found in tombac, an imported zinc alloy.
Keep hunting that site, you've established there were people there in the late 18th century. Bound to be some period coins there too.