Is This A Proto-Coin?

I bought this object today for £1. It is a piece of metal worn irregularly smooth around the edge, stamped on both faces. It is what I would imagine early coins to be like.

Diameter varies from 14.5 to 15.0mm, and is approx 7.4mm thick. Weight 10.2g. It is a blackened metal with touches of a gold-brown colour where worn.

Face 1 has what appears to be a fish with a faint marking above. The gold-brown line is where a line has been gouged across the fish. Face 2 appears to have most of the tail of a similar fish.

Any ideas gratefully received.