I don't know why but the reuse of canceled postage stamps was a large concern in the old days. A lot of people spent serious time trying to come up with the best solution to stamp reuse. However the most bizarre prototype was the embedded explosive percussion cap. This stamp was easy to use just stick on your letter or document then give it a sharp blow. The cap would explode puncturing the stamp.
I wished they would of used this type of a stamp on my mortgage papers, maybe it would of went up in flames.Image: backcap.jpg