Stamperdude - "Peopled use to write messages underneath where the stamp was placed to get around prying eyes, and censors. I think spies may have done this as well. I can't remember where I read this- probaly under the last stamp I soaked off a cover.LOL!"
In regard to this, there is what is called the "Little Alf" story, which is apparently an Urban Legend from the Civil War.
Story goes something like this: a soldier was taken prisoner by the other side and accused of being a spy. He was kept prisoner and was tortured. He was eventually allowed to write home, but only if he let everyone know he was okay because the side holding him didn't want it known that they were torturing their prisoners. In his letter, he told his folks to make sure they saved the stamp on the envelope "for Little Alf".
The problem was, there was no "Little Alf" in the family. When they looked at the envelope, they noticed that the handwriting on the front seemed to slope toward the stamp in the corner. Figuring this all meant something, they steamed off the stamp.
Underneath was a note saying, "They have cut out my tongue."
As I have said, this is an Urban Legend; no one has ever found evidence of its truth. It is sometimes stated that the soldier is in the Union Army, other times that he was a Confederate. It was probably a cautionary tale for spies/captured soldiers to be careful and not fall into enemy hands, or perhaps it was an illustration of hatemongering for the enemy.
In any event, it has an air of untruth; the biggest thing I would wonder is how he got the stamp on the envelope if they had cut out his tongue.
As to stamp placement, this goes back to Victorian times, and was known in many countries. In fact, illustrations of "The Language of Stamps" was a popular postcard subject, as can be seen in this example from Switzerland in 1955:
