This was originally interpreted as meaning that nothing could be written on the address side of a postcard.Frederick Hartmann was planning to market postcards and came up with the idea of the divided back.They are best known for a series of cards depicting historic buildings from Williamsburg, Virginia.This photo studio created souvenir books and postcards from their photographs.
Initially this change did not apply to postcards sent overseas, and postcards printed at this time carry a suitable warning message. Price & Son, The Library, Ware, and shows a feature that appears on some of the earlier divided back cards of the address space being bigger than the message space.
It seems likely that he sent some specimen cards to the Postmaster General to see if they would be approved.
In January 1902 the Editor of Picture Postcard Magazine published a statement, at the request of the Postmaster General, which said that "... the address side, a continuation of the message, or the name and address of the sender, or even an advertisement, so long as such matter does not interfere in any way with the legibility of the address.
Most Real Photo Postcards, abbreviated RPPC, have information on their backs to help in identifying the manufacturer of the photographic paper that was used by the postcard publisher.
If you can identify the paper manufacturer, you can approximate the age of the old postcard.