Well, it's not really a first day cover, but it sort of is to me.
This is the one lot in last week's Siegel's sessions that I was interested in. I wasn't sure what the competition would be like, and as luck would have it I was the only bidder, so I was able to get it for about half of low estimate.
The listing writeup was as follows:
2c Trans-Mississippi (286). Used with two each 3c and 5c Battleship Documentary Revenues on registered cover from New Orleans to Edinburgh, Scotland, New Orleans Jul. 1, 1898 registered datestamp, transited New York with registry label applied over 2c Trans-Mississippi, London registered handstamps, Edinburgh and New York backstamps, Very Fine and most unusual use to pay the 8c registry fee plus double the 5c UPU rate. Despite the illegality of using revenue stamps as postage, this cover passed through both New Orleans and New York without complaint by postal clerks. Ex Dr. Heimburger. Est. $750-$1,000
Illegal uses of revenues as postage on international covers are fairly scarce, and multiples of two different battleship denominations on the same cover even more so. Beyond that, however, what makes it of more interest to me is the July 1, 1898 date. While of no significance to postal history collectors, it makes it a 1st day (of tax) usage, albeit an extremely unconventional one. IMO, this cover is far less interesting to front-of-book collectors.
I have several 1st day uses (of the Spanish American War tax, unfortunately none from the Civil War tax) of revenues on documents, illegal uses of postage as revenues, and even an illegal usage of a proprietary battleship as a documentary, so this makes a nice 1st-day addition... it would be an interesting topic for a 1-frame exhibit... but I don't exhibit.