Here is a nice little advertising piece (5.875 x 3. 375) for the Belles Lettres Series (#50) http://henryaltemus.com/series/series50.htm

It lists 26 volumes in the series at 30 cents each. Since the first 28 volumes were published in 1896, this dates the flyer to 1896.

As I have noted before, I believe that every Altemus Series and every non series book most likely had an advertising piece associated with it. The difficulty is finding them. This is a nice example.