It is probably a hallmark specific to the designer or the company that manufactured the ring. This is typical of custom made jewelry, though it doesn't mean the piece was custom made. I am sure there should be a catalog of registered hallmarks somewhere, but haven't been able to locate anything like this. Maybe a high-end jeweler?
Try this link: http://www.925-1000.com/mexican_marks.html
There is a maker mark from Mexico that looks similar. I am not sure what the general demos on your neighborhood are, but there are a lot of silversmiths in Mexico including those with no marks.
Mexican silver and gold is common around Chicagoland, not sure about your locale.