So, these are not exactly U.S. picture postcards, but...
Coming from my grandfather in-law's estate are two types of unused postcards, about a half dozen of each. I assume they have yellowed with age. Casual examination suggests that these were produced for the convenience of U.S. soldiers in France during and shortly after World War I. On the left are the front and back of one type, another (front and back) are on the right. Both were apparently provided by the Knights of Columbus, presumably for Catholic soldiers.
That's all I can figure. Any additional comments are welcome.