Illegal Use of Revenue Stamp for Postage
This is the image of the upper right corner of a cover that has passed through the U.S. mails illegally. The reason for this is the use of a revenue stamp to pay the first class postage for the letter. These stamps were supposed to be placed on legal documents as proof of payment of federal transaction fees, and were issued during the Spanish-American War. This stamp was used in 1899, after the War. Scott lists this stamp as number R164.
The cover itself is an ordinary business envelope, and shows no special markings that might indicate that it was mailed by a stamp collector, nor does it show any evidence of being intercepted by the Post Office. I find these types of covers to be fascinating, and rarely miss a chance to add new items to my collection.
Note that the stamp is cancelled by one of the many American Flag machines of Boston, Massachusetts. As described in my machine cancel pages, the American company manufactured a large number of machines that impressed the flag cancellation. Collectors find these markings on postal items from the late 19th through first third of the 20th centuries. However, note that Boston was the first major city to use this style of machine cancel.