Based on your photos I'd guess that your brooch was made between 1920 and 1935. Although the pin-hinge is narrow, it was rivited rather than the more contemporary type where the end of the pin is coiled around two dimples stamped into the hinge mount. There was a short window of time when narrow, rivited hinge-pin attachements were explored. To the best of my knowledge the newer type found favor from about 1910 and later however some studios used riveted pin-hinges somewhat later.
I don't know anything about cameos so I can't help you there.
I'd call the design a Victorian revival (I don't know if that term is actually used but that how I'd characterize the design.) as Lindroth was in business primarily during the art deco era, which would account for the pairs of round balls (simulated pearls?) at the top, bottom and sides of the setting.
Here's a little bit about A. L. Lindroth:
A.L.L. Co.: 1922 - 1940s? (Unknown as to last date in business)
The trademark for A.L. Lindroth Company, North Attelboro, Mass. since the 1940s. One item produced was a cameo pin believed to have been carved out of sardonyx/carnelian with yellow gold metal accented with an applied granular texture on the leaves (variegated with yellow, rose, and green gold filling) on each side of the cameo. The pin is considered to be of the Edwardian era (English kings named Edward) and designed for the very elite. Mark: "A.L.L." on the catch. Jewelry by this company is hard to find let alone little is know of the company and it is surmised that the company went out of business in the 1940s.
Lindroth has made some jewelry in sterling silver and marks it as such. They are known to have made a lot of jewelry using chrome plating which is relatively rare for jewelry but might explain what appears to be rust on the filigree and cameo mount.
I hope this helps.
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