For three decades, a set of black-and-white linocuts hung on the exposed brick walls of the Pacific Media Workers Guild office, rarely eliciting more than passing interest. They were clearly well done, and every now and then somebody would speculate about their potential value, or ask about the artist, Leopoldo Méndez, whose signature in pencil could barely be made out on each piece. The late Rex Adkins, a San Francisco Chronicle editor and longtime union officer, told anybody who asked that he had bought the prints at a nearby pawnshop for $1 each. He had them framed, spending more on the frames than the art. Nobody knew anything in particular about the prints, or the artist, or the story behind the scenes depicted of peasants in struggle, including a series showing a captive rebel facing a firing squad. For an embattled labor union with longtime ties to the United Farm Workers, the low-budget wall decorations seemed to make a good fit. Now, it turns out, the art has become an asset worth a lot more than mere decoration. Photo: A 1950 linocut by Leopoldo Mendez.