Identifying a ring I found

I found a gold ring, it has 3 stones set into a silver setting. It has a gold band and inside the band the markings are these. S or could be a 5 followed by full stop then LTD then a circle with symbol inside of it then 18<+2 then on the opposite side it looks like a 4.B. So this is it 5 or S.LTD O18<+2. Then 4.B. As this is very old the D could also be a K.

Hope you can help me. Pauline