Reference Source Needed

I need a good reference source for the variety, census (if any), and scarcity of cancels for the Panama California International Exposition at San Diego (not to be confused with the World's Panama Pacific Exposition in San Francisco).
Thanks!

Get a copy of William Bomar's "Postal Markings of United States Expositions" It contains illustrations, known date ranges, census data on the rarest markings, and price estimates.

Here's one for the San Francisco Exposition. Some Southern Cal advance publicity.

Thanks for the reference source John. Phil, I've several of the S.F. Expositions but hadn't seen the particular San Diego cancel that I posted. Thought I'd research it and need a source.

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.