Only think I have found is Netherlands letter dates from 1863-1953. There are a few different styles the correspond to specific dates. See the table at address below. Although an A inside a Circle does seem like a potentially common mark, I haven’t found it in any other searches.
That has the same markings "18KHGE"with an encircled capital "A". It was my grandmother's and she was born in 1923. Not sure when she acquired it or how because I found it after she died. It was difficult to make out all of the letters bc they are placed in an awkward spot. Thanks to this post I was able to put it all together.
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A in a circle by: Anonymous
I’ve been looking for the maker of this mark too; the capital A in a circle of an 18KHGE ring. It is NOT Ana Bekoach. Ana B’koach is a prayer, not a person. Has anyone found the A in a circle maker yet?
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"A" in a circle= Ana BeKoach Kabbalah Jewelry by: Eileen
A capital letter A inside a circle indicates the maker's mark of Ana Bekoach, Israeli Kabbalah jewelry. The pieces have heavy gold electroplate.
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Same by: Anonymous
I have a few estate pieces stamped 18kgp, followed by what looks like a capital a. Searching the world encyclopedia I found a makers mark to show that the ring is accompanied by a lozenge or barrel cut stone, which is true of one of the pieces, the other, not so much.