Al, the more specific answer to your question ... In additional to handstamps used at the Expo Station, there were numerous advertising slogan machine cancels throughout California including machines made by American, Columbia, International, and Universal. Yours is a Columbia machine cancel, known used in 6 cities. The machine in Riverside is listed (in an advance copy of the 1996 edition) as March 9, 1916 - December 28, 1916. So the known date range may be longer now.
Here are 4 of the 6 Columbia's.