1. This rule was used to relate horse power, speed and weight of a car. It was very simple to use. Although not an automobile engineer I would guess that this rule makes an implicit assumption about engine efficiency and aerodynamic drag using values which were appropriate at the time.
2. The rule was copyright by Charles C Ettien. Interestingly I believe I have found his obituary at http://www.obitcentral.com/obitsearch/obits/ca/downey/ca-downey10.htm . He died at the age of 88 in, probably 1998. (His obituary does not give a date of birth or year of death but other obituaries on the same page can be traced to that year.) He lived at Downey in California at least since his marriage in 1941. He worked on water projects, then he and his wife ran a paint and wallpaper store and he also operated as a painter and decorator and when semi-retired was chief maintenance engineer for the Lincoln Hospital in Los Angeles . However the obituary also states:
"He loved bowling, and was an avid target shooter (even loading his own shells), and loved "muscle cars." He raced a 1954 Ford pickup with a full-blown Chrysler engine at drag strips in the 1950s. His last "muscle car" was a 1966 Dodge Charger equipped with a 426 hemi engine. "
So, given his technical background and his interest in drag racing we can be fairly certain that he was also the slide rule designer.
3. After the above had appeared on this web site, I had an email from Charles Ettien's daughter. She confirmed that he was indeed the inventor of the rule and he had been very disappointed that his rule never caught on. He seems to have been a remarkable man. He only finished 6th grade at school (equivalent to the first year at secondary school in England), taught himself calculus and trigonometry from text books in his 30s and after retiring from his retirement job at Lincoln Hospital taught himself to become a locksmith.
My thanks to Joyce Fanning, Charles Ettien's daughter, for the extra information.
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