Had this one in for about 20 years!! Any Ideas on how this "Raised X" got on this coin. Do you have a explanation?

I've had some real nice pieces, from a pair of 1975D Lincoln Cents matched set, to a Lincoln struck 5 times. Going through Mint bags, getting anything from a Clip to a Muti-Strike. Not knowing what might be in there. It's a strange feeling seeing 5000 lose copper colored coins lying on a table top.