Copper disk with strange shank (?)

[keithdetects]

I found this over the weekend at a site where I have found tombac buttons in the past. When it came out of the ground with the "shank" down, I assumed it was a large copper. Then I flipped it over and assumed it was a large button. After getting the dirt off, the "shank" doesn't look like a shank at all. Any ideas? Thanks!

 

[Dave_e]

Is the inside threaded?

Looks to me like it could be a corroded military style collar disc.

 

[hunter1]

I have something like that in pewter, but the shank is broke off.

 

[keithdetects]

Is the inside threaded?

Looks to me like it could be a corroded military style collar disc.

There doesn't appear to be any threading on the inside.

 

[wolf-n-wa]

I'm thinking leather rivet!