@BP Addict ,
Your question is a good one. You are not the first to take an interest in that enigmatic advertisement, and its reference to the "California Rifle."
The St. Louis Hawken shop got rolling in the early 1820's. The sturdy, iron-mounted rifles which most of us think of when we hear the word "Hawken" were called "mountain rifles" in their time. Note that the ad by Hawken & Campbell from the January 20, 1855
Missouri Republican, cited by Hanson, says "Mountain and California rifles made to order, and repairing done on the shortest notice." So, California rifles
were made in St. Louis, by the Hawken shop, and they were evidently distinct from the "Mountain Rifles." The term, "Plains Rifle" had not been coined yet, and was apparently not used until into the 1950's.
A few years ago, this brass-mounted late S. Hawken rifle surfaced:
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I think this is probably a "California rifle." It is a sturdy, big bored half-stock, similar in many respects to the more familiar Hawken mountain rifles, but is mounted in brass rather than iron, has that "hook & scroll" triggerguard, and only one barrel key. Otherwise, it is pure Hawken. So, I think this was just a style of rifle that came into vogue in the 1850's, appealing to people who were heading to the Golden State to seek their fortunes. Reading on a bit further in the Hanson book, you'll see some references to gunmakers in Philadelphia, who were building guns specifically for the California market. One of the best known of these was Wurfflein. The triggerguard on this Wurfflein rifle is different from the Hawken shown above, and it has two barrel keys, and a cast nosecap. However, it would likely fit in the same niche in the firearms market of those days:
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This one, built by Wurfflein for Major James Carleton, may be more true to the "California" type, and even has the same triggerguard as the Hawken shown above.:
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As one of the fellows mentioned, Slotterbeck was a California-based gunsmith who turned out some fine rifles, which were similar to the ones shown above. However, there were plenty of others. This one was by a California gunsmith named Bigelow:
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So, I have no definitive proof, but I think it is likely that the brass-mounted late S. Hawken rifle shown at the top of this gallery is a Hawken "California Rifle." It is similar in appearance to the Philadelphia rifles made specifically for the California trade, and to the late percussion rifles made by gunsmiths in California.
My opinion, for what it's worth.
Notchy Bob