A routine landscaping project in San Francisco has turned into an art-world mystery after a couple unearthed a carved limestone sculpture in their backyard that they believe is linked to the city’s storied Art Deco past. Greg Gatwood and Judy Gitelson have several theories that hint at the sculpture’s possible connection with one of the city’s most influential artists, Ralph Stackpole. Stackpole, a prominent figure in the San Francisco art worldduring the 1920s and ’30s, created major public works including statues for the old San Francisco Stock Exchange and murals inside Coit Tower. If the piece turns out to be…