That's the crown but there is no separation between the marks and #s by: Rich
And no sign of silver, a magnet will not stick to it.
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Crowns, 18, letters on US vintage rings. by: Anonymous
it sounds like these marks might be pseudo-british hallmarks. Hallmarks with variations of crowns and 18 and 9 ct and letters following like a date year stamp give you the feeling it's British made. Crowns and Britain well...you know what I mean. The 18S could mean it's vermeil, which is gold plating over Silver. You should see tell tales signs of this, the color will look faded or a different color altogether at the wear points.
The UK 18 carat gold hallmark used from 1798 - 1974 looks like this image:
