Do you know what this is?

[Barbara]

Found this in the u bend of kitchen sink whilst retrieving a screw from a cafetiere which had fallen down the plughole.I think it's made from copper or brass and weighs about 13 g.There's no thread on it and the raised metal dot doesn't move. It may have been there a while.Puzzled.
Thanks.

 

[SimonG]

It's the centre of a strainer waste plug.

 

[Barbara]

Ah..so it is...obvious now.It must have been there many years as it's from the half sink plug which broke ages ago.
Thanks Simon

It's the centre of a strainer waste plug.