To legally qualify for the gold filled stamp in Canada the weight of the gold layers had to be at least 1/20th of the total weight of the metal in the sandwich of gold and base metals. The patent for "gold filled" metal was granted to J.Boss in 1859
As an aside, you often see Americal watch cases stamped in "years" ie 10 years, 20 years, 25 years. That was a sign of the thikcness of the gold layer. The case was warranted by the manufacturer to not wear through to the brass, in that many years (in normal use).
In 1924, the US government ruled that practice illegal, leading to a swith in labelling to 10K or 14K gold filled, as applicable.
I have a couple pages of diagrams that show the variuos case grade and maker marks, those will be in my guide, as well as case types, and how top properly handle a hunter watch case (hint- NEVER snap it shut)
Cheers
Harold