Hello and welcome.
From your description, it sounds to me as if it were a normal British silver coin that has had one side ground off, and re-engraved with a memoriam to the king after he died. If you tell us how large it is, we could tell you which coin it was made from.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis