What the finder described as a "hollow lead disk that fit inside a concave brass piece" is a clock pendulum weight. The pendulum's metal shaft, which was shaped like a miniature long 2x4 board, fit through the tunnel in the disc. Yours is a very simple, non-ornate version. The photos below shows some ornate ones, and some plain ones like yours. They date from the mid-1800s into the early 1900s, after which time, most "wind-up" clocks were superseded by electric ones… but some are still being made today, for wealthy folks who like antique-style clocks.