Hi Andrew,
 
My guess is some kind of fishing net sinker though this is a very small
one.  In Saskatchewan, Canada, where I come from, a friend of mine has
been studying prehistoric (about 700-1,500 years old) net sinkers.  They
are usually more rotund in profile and larger, but have the same kind of
concentric ground rings.
 
I see you have two dollar coins as we now have in Canada, introduced
just a few years ago (the 'twonie' to go along with the 'loonie', our
one-dollar coin from 1987 onward which has a loon on the obverse).
 
Larry Buhr
University of Nevada, Reno