I happened to see a brief story in the Chronicle this morning, about an old sign that was recently uncovered when the Bayview branch of the public library was demolished for rebuilding. The previous structure had been there since 1921, so it's at least that old. I sent Scott out to snap a picture, before the new branch library goes up, and covers the sign for another century. The Chronicle had an interesting conversation with the SF Planning Department's Dan Sider:
"I don't know how we would classify that sign under the Planning Code," he said with a laugh. "It isn't really advertising. Maybe I would call it art?"
The city recently decided they wanted to protect vintage signs after an uproar over an old Coca-Cola sign in Bernal Heights. Planners found that sign to be illegal and ordered it painted over, until the Board of Supervisors created a new process to protect vintage signs.
The Vintage Workwear blog notes that the "N.B. Products" banner, in the corner, refers to Neustadter Brothers, a workwear manufacturer that eventually came under the ownership of Lee Jeans. They also link to a photo of the area, from 1928, wherein this sign is obscured by the adjacent storefront, below another sign for Ed's Shop. Elsewhere on the web, I can find pictures of Neustadter Brothers' office buildings on the 100 block of Sansome, and at 1st and Mission, in San Francisco; and a picture of one of their Boss of the Road ads, from a 1905 Portland Fire Dep't Yearbook.
According to the archives of the USGenWeb project, "Neustadter Brothers first began business in San Francisco in 1852, the original partners being Louis and Henry Neustadter. They prospered from the beginning, the business growing in volume and importance, and in 1900 the company was incorporated in California and Oregon, with a capital stock of one million dollars."