P & C Tools was a trademark registered to Plomb and later Proto tools of Chicago. The initials are from the founders John N. Peterson and Charles F. Carlborg, who started the company in Boise Idaho in the early twenties. The whole story is here at Alloy-Artifacts. I haven't been able to find what the Clik-Stop refers to or if it ever actually had a patent issued.
Not an authoritataive answer, but having held one in my hand I can tell you the Clik-stop refers to a catch or detention on the adjuster thumb wheel. I've never taken one apart to see what mechanism provides the detent action.
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