Non magnetic metal disc

[strycker]

Found this today in a field metal detecting along a spring feed stream runs to a pond. Approx 1/8" thick. Looks like stone but it is metal. Angled drilled holes in center. Pics are front and back as flipped on its axis. It is also fairly heavy but not iron. Any ideas? Found a half horse shoe nearby as well. Thought it was a button at first but once dirt fell out of center I was puzzled.

 

[Charlie P. (NY)]

Looks like a lead plug that has been pried out and then oxidized.

 

[strycker]

Looks like a lead plug that has been pried out and then oxidized.

What would this have belonged to originally if it was a plug? Seems big.