First, be sure the stamp doesn't instead read STER (sterling silver). I know
that sounds too obvious, but sometimes...
Then go to the U.S. patents and trademarks office web site and see if you can
look up that trademark. Their records on line don't go all the way back, but if
the earrings are U.S. manufacture and relatively recent, that may do the trick.
If it doesn't, find a good library that has patent registration materials.
Trademark registration info is published and goes back much farther than what
the patents and trademarks office has digitized and available from their web
site.
Peter