The markings allowed museum director Mary Levkoff to determine the painting was created by Spanish artist Bartolome Perez de la Dehesa in 1690, The Tribune of San Luis Obispo reported this week.
"This is a major new discovery for the oeuvre of Perez," Levkoff told the newspaper.
The painting depicts the Annunciation - the angel Gabriel announcing to the Virgin Mary that she will be the mother of Jesus.
The work, which is about 8.5 feet (2.6 meters) high and 5 feet (1.5 meters) wide, hangs in the Assembly Room of the main residence of the elaborate estate built by late newspaper magnate William Randolph Hearst and filled with his extensive art collection.
Records showed that Hearst bought the painting in 1927, but officials of the Castle, which is now part of the California state parks system, knew little else about it until last fall.
In November, tour guide Carson Cargill was leading a group through the Assembly Room when sunlight reflected off a mosaic floor and onto a portion of the painting not usually so vividly illuminated. The light revealed dark letters on a deep brown background.
"At that point, you could see it," he told the newspaper.
When the tour was over, he and another guide, Laurel Rodger, looked closely and then took their findings to Levkoff, who began doing research that included consulting an expert on Spanish Baroque paintings.
Hearst accumulated 250,000 acres of ranch land on the scenic coast between San Francisco and Los Angeles and spent decades building what he called La Cuesta Encantada on a hill with a commanding view of the Pacific Ocean.
The 68,500-square-foot (6,364-square-meter) main residence, which he called Casa Grande, has 38 bedrooms, 42 bathrooms and 14 sitting rooms. Several other guest houses have a total of 46 rooms. Extravagant indoor and outdoor pools as well as gardens complete the estate, which is now a major tourist destination.
Information from: The Tribune, http://www.sanluisobispo.com
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