Though errors on stamps are usually rare and expensive, there are luckily few exceptions. The United States “Dag Hammarskjold invert” error of 1962 was the first invert error to occur on any United States stamp since 1918 Inverted Jenny. And AFAIK, it is the second deliberately mass-produced error stamp worldwide (the first one I believe is DDR 5-year plan 20/24pfg definitive stamp).
The 4c (standard letter rate at the time) commemorative honoring Dag Hammarskjöld, late Secretary General of the United Nations, was issued on Oct 23 1962. The stamp was printed on the Glori Press in a yellow, black and brown design on white paper. The print run was 121,440,00 stamps.
The invert error and reprint
In early November 1962 a New Jersey jeweler named Leonard Sherman received stamps containing inverted center. The invert had occurred when some sheets of stamps were fed into the press backwards. At first there existed only 400 copies of “Hammarskjold inverts”.
But the Post Office Department did not wish to produce rarities to collector markets. In lead of postmaster general J. Edward Day, a special printing of 40,270,000 Dag Hammarskjold stamps identical to the invert errors was ordered. The reprint was issued to the public on 16 Nov 1962.
The value of newly found error melted away as there is no way to differentiate reprint from original error. The only exceptions are very few inverted copies having (clear) early date cancels and first day covers with inverted stamps – these are very scarce and fetch high prices.
Subtypes of invert
What many (especially non US-collectors) may not know is that there are 3 subtypes of the invert. The subtypes come from the the first vertical row of the pane where different position of the pane lead to birth of these varieties.
With the first subtype the width of white stripe on the design is 3.5mm, with second subtype it’s 11 – 11.5mm, and with third subtype it’s 9.75mm. Unfortunately I don’t have these subtypes to show (besides the first and most common subtype, see image above), but they can be seen on Paul Hennefeld’s collection.
Scott stamp catalogue does mention the subtypes, but AFAIK does not list them separately. Michel stamp catalogue on the other goes all the way, and lists and values the subtypes. The most common of these subtypes is IIa; the other subtypes are far more difficult to find due to much smaller print numbers (which is why they have a double of value in Michel).
PS. The stamp does contain one more error (though nowhere as interesting)… As Mr. Hammarskjöld was from Sweden, his lastname contains umlauts (very common here in Scandinavia). As all can see the stamp design does not contain them. I think these were left out for convenience of US speakers.
You might also be interested of related posts about Stamps and postal history of USA.