Posted By:
David'How to Date a Cigarette Pack'
http://cycleback.com/packdate.html
Of extra note is that, when too many stamps were bought by the ciagarette manufacturer for sealing, some could be left over and used for future use. Thus, a '1909' tax stamp could be used on a later date pack.
Leon is correct that the red overprinting is very important. When it is underneath the wax-like paper it is very difficult to impossible to read-- and I would beleive the seller if he/she claims that he cannot read it. In many cases, the only way to read the red overprinting is to remove the wax paper, which may or may not be desirable.
Casually, the Sweet Caporal packs from 1909 to at least the early 1920s have virually the same graphics style.