Seems like it should be me asking you, instead of the other way around. :icon_scratchchin: I've always assumed ~ 1880. That's when the 1st series Erie skillets are dated from. 1880 is also the year of the earliest Griswold patent I know of, a Selden & Griswold button hinge waffle iron.
Then there is page 10 of the blue book, which shows an ad from a 1954 high school yearbook, titled "Griswold marks 89 years of progress". 1954 - 89 = 1865. The ad describes and shows "our original cast-iron ware set", containing, among other implements, a toaster, long-handled waffle iron, a large pot, a long-handled skillet, and a broiler, all for use over a open fire or on a hearth.