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The letters, PLO could be a unique maker's stamp. If your cameo also features UK-style hallmarks , such as, a crown, a city letter, or an assay date letter, these hallmarks can help narrow down its exact origin and date.
Although the photo isn’t clear, it appears your cameo features a detailed, high-quality hand carving of a woman's profile, on authentic material such as carnelian shell, sardonyx, or lava, therefore , the value of your cameo as an antique or vintage piece outweighs the value of its scrap melt 9 ct gold frame.
