Stamp boxes on printed or lithographed cards also offer dating clues. Often there is a reference in the box to the amount of postage required. Of course, if the card is used and has a stamp, that too gives a clue, both by its value, and the style of stamp itself.
For U.S. post cards, the standard postal rate was:
1872 1 cent 1917 2 cents I found this on a website..how to date a card..it has on the back place a one cent stamp for US and 2 cent for foreign in the little stamp box..hope that helps..