Antique clock pendulum-weight. The shaft on it is somewhat bent out of its original shape.
The photo below, borrowed with permission from a civil war relic dealer's website, shows you are not the only person to incorrectly guess this disc is a horseharness rosette. But note the raised rectangular "attachment fixture" on the sheetbrass disc's lead-filled back for the shaft.
[Good ID! That one still has the adjustment.QUOTE=TheCannonballGuy;6719990]Antique clock pendulum-weight. The shaft on it is badly bent out of its original shape.
The photo below, borrowed with permission from a civil war relic dealer's website, shows you are not the only person to incorrectly guess this disc is a horseharness rosette. But note the raised rectangular "attachment fixture" on the sheetbrass disc's lead-filled back for the shaft.[/QUOTE]
Joe, I found a photo in my relic-images folder that shows a group of non-excavated antique clock pendulum-weights, one of which seems to be a match for yours. It is slightly to the left of the center of the group.
Some of you other folks might want to Save this photo, which can be helpful for future identifications, because it shows many other variations of antique clock-pendulum weights.