Coin Collecting

I have a Guernsey 20p coin dated. It’s normal apart from the ‘S’ stamped into the queen’s head on the reverse. – the ‘S’ is engraved rather than being ink or anything.

This is certainly not normal as I used to live in Guernsey. Do you know why this coin may have come to be engraved? Does this make it special in any way?

Many thanks in advance.