Quote:
In this case, the stamp actually fulfilled a postal function. As the article explained, the use of postage stamps as a receipt for a postage due bill payment is an approved postal service procedure. Though with full gum on the back the stamp resembles a CTO, it is, in fact, what is known as a favor cancel.
Something like this might explain some cancelled postage due stamps with full gum that I found in a mixture some years ago.