I believe the edition number (tenth) was chosen to coincide with the year of publication (1910) and doesn't mean that there were nine previous editions.
 
 

Note the mention of a Frederick Post office in San Francisco; that office has an interesting history: 

     "1918: Frederick Post Co. opens a branch in San Francisco and puts Frederick's nephews, Rudolph and Victor Post in charge. This branch has no blueprint shop.

     "1921: Rudolph Post recruits Jim Dieterich from K&E and obtains financing for a blueprint operation from his uncle, Charles Bruning. Later in the year, the two buy out Bruning's interest and form Dieterich-Post Company."

The Dieterich Post Company was still operating in California in 2010 and its website gave the above information.  The Dieterich Post website also stated that 2010 is its 93rd year which implies a startup date of 1918--that's earlier than the 1921 date given in the quote.  (By 2011 Dieterich Post had changed its name to American Reprographics Co and all historical information had been deleted from its website.)

I have a 1921 catalog (first edition) from "The Frederick Post Company of California."  It makes no mention of the Frederick Post Company in Chicago and lists an entirely different array of products than the Chicago company's catalog.  It is clearly a totally different entity from the Chicago company.

The Frederick Post Company (Chicago) never mentioned a San Francisco office in any catalog between 1910 and 1960.

So here's what I think happened to the San Francisco office of the Frederick Post Co:  It was opened no later than 1910.  Sometime before 1921 (likely 1918) it broke free of the parent company and operated as "The Frederick Post Company of California."  Sometime after 1921 it changed it's name to Dieterich Post Company--likely when it got a letter from Frederick Post's attorneys about using the Frederick Post name.

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Note: The information about Rudolph and Victor Post and Jim Dieterich is part of a history of the blueprint industry in the United States that originally appeared on the Web at 
http://www.wide-formatimaging.com/pages/issues/2000/600_repro-roots.shtml but that page had disappeared as of April 2011.  It was reprinted in the ISRM but I can't find it there anymore, either.  The American Reprographics (Dieterich Post) Website, which no longer carries any historical information,  attributed the information to the ISRM.  Fortunately, I made a copy of the original "wide-formatimaging" page before it disappeared.