Check letters were employed first on the 1840 Penny Black and Twopence as a measure to frustrate counterfeiting. The check letters are located at the south-west and south-east corners of the postage stamps within check letter boxes. The design of the stamp, including the portrait and engine turned back ground, and star corners, were impressed through the use of a die upon the plates. The check letters were then later impressed by hand through the use of a punch. Letter combinations were used to complete a series of pairs of check letters for 240 positions which was the standard size of the sheet at the time of press. The size of the sheet would be further reduced for distribution to the post offices. The scheme called for a combination of letters beginning with the first stamp of the first top row being "AA." This scheme applied to the following rows through the final row beginning with "TA" and to the final stamp position on the sheet "TL." The diagram below illustrates a portion of the sheet lay out of the check letter system (see figures 1 and 2) in a schematic format:
|A A||A B||A C||A D||A E||A F||A G||A H||A I||A J||A K||A L|
Check letters AA through BL
The rows continue through alphabet coming to completion with the last row:
|T A||T B||T C||T D||T E||T F||T G||T H||T I||T J||T K||T L|
The last row of the check letters at the bottom of the sheet: TA through TL
Since the check letters were punched into the check letter squares by hand punching, it follows that they were not entered dead center each time, but more often placed in different and unique positions each time, they are useful to the Plater. The letters were sometimes very badly placed and are located in the top of the square or bottom of the square. Sometimes they were placed at the extreme right or left of the square. If they were placed at the extreme left or right of the square the may have skewed the square as is the case of Plate 2, position PI see figure 3).
Penny Black Plate 2 with skewed corner square. This occurred during the printing of the plate in red ink
Defective or faint letters:
Sometimes the letters were not punched in very deeply and appear faint from the beginning of the plate's use. Also, when the plates were impressed sometimes raised metal formed around the impressed areas, creating burrs, which were burnished off the plate. In some cases parts of the letters may have been accidentally effected by the cleaning process and became faint or defective. yet another case for a faint letter is if the plate was repaired by re-entry and the die (which had blank check letter squares) pressed down upon the punched in check letters flattened out the check letter and made it less visible.
This occurred when repairs were made before the plates went to press. Apparently at the time the check letters were punched into the squares, the letters were badly placed or the wrong letter was placed in the check letter square. The process was very closely followed in an attempt to maintain a high degree of quality, and very few major mistakes are to be found on the plates. Perhaps the only major exception is the famous "B" Blank stamp from Plate 77, in which the "B" of position "BA" was accidentally left out of the B check letter square and put to press with the check letter square empty. It was later discovered and the "B" was put into place. In the case when the wrong letter was placed into a square, the incorrect letter was burnished off the plate and the correct letter was substituted. However, sometimes the impression from the incorrect letter was not completely removed and is visible along with the replacement letter. The incorrect letter is not entirely visible but only some small part of it which escaped the burnishing process. An example of a typical double letters is shown in figure 4.
Plate 2, position FA, double check letter (note the portion of the previous letter underneath the F)
Some letters are characteristic of a certain plate or plates. Plate 10 for example, has a unique long tailed "R," a short squat "H" and" T," and a long" I" and a tall somewhat slender "N." PLate 2 has a broad "D." Plate 11 has a unique letter "J" with a large ball on its foot. Plates 9, 10, and 11 have a similar type of square "J." The square J reappears on most but not all of the Red Plates from Plate 12 through Plate 45. A further variety of the "J" is the "J" Flaw which appears on Plate 10 and a number of the Red Plates as well. it was caused by the check letter punch being held at an angle when the letter was struck into the square. A part of the punch which was not intended to make a mark on the plate left a slight impression on the right side of the "J." See figure 5.
The "J" Flaw (note the small mark to the right)
"E" is another letter helpful for identifying the Penny Black plates. On Plate 1 it is a large type with the bottom serif often extended to the left. On Plates 5, 6, 7, 9, and 10 it is often a narrow type, and on Plate 8 it is small.