Does it have 2 -3 grooves on the Reverse. It might be a printing plate ( type Plate ) for the Towns Stationary. I have some type plates in my garage. You set these up and use an ink Roller to cover the plate and then you pull a roller to make the impression on the Poster Board.
I'm assuming you are in San Francisco. This is a nice piece of The City's financial history. This was the bank founded by A.P. Giannini in 1904, that he grew into the Bank of America. America's Banker: A.P. GIANNINI - TIME