Battleship Revenue Used As A Patriotic Label/Cinderella

I picked up this cover on ebay. At first glance it looks like an improper/illegal use of a battleship revenue as postage, but I don't believe it is.

It is a Spanish-American War patriotic cover sent to Germany via steamship S.S. Augusta Victoria. I believe the correct rate for the cover would have been 10 cents, so the revenue stamp was not needed for postage. Instead, in keeping with the theme of the cover, I believe the intent was as a patriotic label, i.e., "Remember the Maine!" (the ship depicted on the revenue stamp is the U.S.S. Maine, which was sunk in the Havana, Cuba harbor on February 15, 1898).

Some scuffs on the front, but still a unique and interesting cover.

Standard UPU rate of 5 cents per half ounce, thus rated as a double weight letter with the Battleship revenue going along for the ride.