While my primary collecting focus is revenue stamps of the 19th century, and I give 20th century material comparatively short shrift, there is one stamp (2 stamps actually) that have piqued my curiosity: Scott # R733-734, the last two documentary stamps issued by the U.S., issued in 1962 and 1963 respectively.
This discussion focuses primarily on Scott #R733, as it generated the most hoopla and collector interest, whereas the subseququent #R734 a year later (removal of the text "ESTABLISHED 1862") was pretty much disregarded by the collecting community.

From Ron Lesher's 2018 article in the
American Stamp Dealer & Collector:
Quote:
There is an example of a revenue stamp with a release that was marketed in post offices like commemorative postage stamps – the 10¢ documentary stamp that was issued in 1962 commemorating the centennial of the creation of the Internal Revenue Service (Scott R733). The stamp looks like a contemporary commemorative postage stamp and was printed in two colors on the Bureau of Engraving and Printing's Giori press. To promote this event, a poster was created to be placed on post office bulletin boards, and the stamps were widely available in post offices. This marketing of a revenue stamp to postage stamp collectors appears to be unique among the revenue stamps of the world.
Since these stamps were available to the public, and stamp collectors are gunna do what stamp collectors do, a range of first day covers both with and without cachets ensued. This was, of course, exacerbated by the fact that these stamps were NOT valid for postage. So all manner of creative ways of accompanying postage were devised.
The stamps commemorate the centennial of the creation of the Internal Revenue Service (IRS) on July 1, 1862. The 100th anniversary date fell on a Sunday, so the majority of covers have a July 2, 1962 date.
I have been slowly but surely acquiring various and sundry examples of these covers. One cover in particular that Ron showcased in his article caught my fancy, as it crosses over into one of my other collecting interests: the half-cent postage dues on cover.
I figured if one example existed, there were likely to be others, and so I patiently waited for one to appear on the market, and now several years later, one did, and I pounced!
Instead of affixing 4 cents postage along with the revenue stamp, this particular collector/creator got the post office to affix and cancel postage dues, in this case a block of 8 of Scott #J88, the half-cent postage due.
The cover below, while philatelic by definition, is also an in-period use of J88. Even philatelic uses of J88 are quite scarce.
Regardless of its actual status or validity, the cover is a lovely bridge across the two collecting areas.
