Posted to Twitter by historian Michael Beschloss, without explanation. He includes the hashtag #WPA, so maybe he came across the picture while doing some work on the Works Progress Administration, which didn't come along for another three decades. Click on the photo to see it even bigger.
Next day update: Nice work by history researcher Larry Harnisch, who traced the photo to the California Historical Society Collection online at USC. He says it's marked circa 1906 and shows Broadway looking north from 6th Street. Larry identifies several of the buildings to pin down the date. Check it out