"the earliest THEATRICAL poster I know of (or can think of off the top of my head) is from about 1910"
There have been some for specific movies from 1907. Also, there are undated ones that go back further that are undated, but the movies are from before 1907, and they are first release posters.
There is a stock poster with a printed inset for "Watering the Gardener" from before 1900.
None of them are worth as much as the rare Buehler, and ALL are less desired than even the common Buehler!
Bruce