I don't believe the coin is
plated.
I think that you have a silver coin with several things happening.
Though the whole coin is silver, the surface and interior are slightly different due to some or all of - deliberate surface enrichment at the time of manufacture (e.g. pickling in a natural acid), burial, and cleaning. The surface layer is then cracked and flaked off in parts revealing a darker looking, but
still silver interior.
There is also some build up of horn silver(?) on the
reverse in particular.
SC