Recently I came across a post on the PSE website of the following plate block.
It is a plate block of Martha Washington, Scott #556, from the 1922-1925 regular issue and describe by PSE as having transitional perf. 10 on the bottom right stamp. A #556 plate block of 6 has a catalog value of $250 hinged. However Scott includes a listing of #556b as a perf. 10 top or bottom with a catalog value of $3,500 hinged. PSE did not describe the lower right stamp in this block being perf. 10 but as having a “transitional perforation”. Being unfamiliar with the term transitional perforation, I did a little research. I found a definition in the glossary section at the United States Stamp Society website as being when a stamp is partially perforated 11 and changes to perforation 10.
To review, the perforation gauge is the the number of perforations per 20 mm. So a perf. 10 stamp has 10 perforations per 20 mm. For stamps with compound perforations, the standard practice it to describe a stamp’s perforations with the horizontal value followed by the vertical value. So a stamp that is perf. 10 by 11 has 10 perforations per 20 mm across the top and bottom and 11 perforations per 20 mm on both sides.
Perforations are made by rolling a perforation wheel across the imperforate sheet. The perf. 10 errors arise from replacing some of the perf. 11 pins with perf. 10 pins on the roller. In the 1922-1925 regular issue series, the perf. 10 error is also found on Scott #562c and #568c. 562c is a great rarity with a catalog value of $85,000.
I found another example of a transitional perforation plate block from a recent Siegel auction.
Siegel describes this block as Scott #498 Top plate no. F 13381 block of six, bottom stamps show transitional perforations with right stamp Perf 11, center stamp transitional Perf 10/11, left stamp Perf 10. The change in perforations across the bottom clearly shows that the perforation gauge changes from 11 to 10, thus the term “transitional perforation”.
In addition to #498, the 1917-1919 unwatermarked perf. 11 Washington-Franklin series shows a lot of examples of the perf. 10 error. Scott numbers with the error include 498g, 502e, 506a, 507a, 508c, 509a, 510a, 511a, 512b, 514a, 515d, 516a, and 517c. All these errors are very rare with high catalog values.
These perforation errors are very hard to spot. I can generally tell a perf. 10 from a perf. 11 without using a perf. gauge but I could not with the two blocks shown. So there are likely more of these errors hidden and unidentified in collections.
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