Uk 123

Hi, my first post on here. I’ve collected coins for a number of years, but recently came into possession of a George 111 third Guinea which used to belong to my dad, and I’d only ever seen it when I was younger. It has a marking that I always assumed was a hallmark, but can see now it’s not! Does anyone have any idea what it is? Has somebody just decided to put a mark on it at some point. Would be really interested to find out, but I really like the coin as it is, despite this marking.