9. Scandal! I found a newspaper article from 1893 that documents a court case involving this company, F G Norman & Sons. This company, along with nine other construction companies, sued the "French Church" for not paying its construction bills. The church, Notre Dame des Victoires, is still standing at 566 Bush Street. This sewer vent cover is at 20th and Valencia streets.
10. The only reference I could find to the "Warburton, plumber" stamped on this sewer vent cover at 19th and Valencia streets was a directory listing from 1862. That makes this plumber the earliest referenced company, or person, of any I've found to date.