By Michael Baadke
Printing plates and printing cylinders used to print postage stamps in the United States have long been identified by numbers, or letters and numbers combined.
That way, if someone at the printing plant notices a problem with the printing of a certain stamp, the plate number can be used to locate the proper plate or cylinder so the problem can be investigated.
The numbers also help the printer with record-keeping, particularly if more than one plate is used to print a single stamp design.
A plate may be used to print a large sheet of 80 or 100 or 200 or more stamps, but the sheet is usually sliced into smaller individual panes of (for example) 20 or 50 stamps.
Because of the way the sheets are divided, the plate number is printed several times on each sheet so that it appears at least once on each of the smaller panes that are sold in post offices.
Over the years plate numbers have appeared centered in the margin or printed in one corner. On modern stamps the numbers often appear in all four corners of the pane.
Figure 1 shows as an example the upper-right corner of a pane of the 5¢ Cordell Hull commemorative stamp of 1963. Plate No. 27615, shown in the illustration, identifies one of the four plates used to print this stamp.
Instead of saving just one new stamp when it is issued, some collectors save blocks of stamps that include the plate number information printed in the attached selvage. These items are known as plate blocks or plate number blocks.
Plate blocks often consist of four stamps with all of the surrounding margin paper attached, including the plate number information.
One fairly recent example is shown in Figure 2. The 32¢ Henry R. Luce stamp, Scott 2935, was issued in 1998.
The plate number for this example, shown to the left of the lower-left stamp, is "B1."
In 1980 the United States Postal Service changed its numbering system for printing plates.
Where previous issues had shown a sequence of several numbers that were used only on one specific plate, new issues from that point on would often begin with a number such as "11111," with each digit in a different color.
The number of digits in the plate number usually indicates the number of colors that were used to print the stamp design. A plate number with five "1s" means five colors were used in combination to print the stamp.
If additional plates were needed, one or all of the digits might increase by one number, so the next plate combination might be identified as "22222."
The single-color Luce stamp only needed one digit for its one color. The letter B before the number indicates the contractor for the stamp, in this case, Banknote Corporation of America.
The Scott Specialized Catalogue of United States Stamps and Covers regularly publishes a listing for each new U.S. issue that includes a description of a plate block for each new issue.
The Scott description for Scott 2935, the Henry R. Luce stamp, is shown above the Luce plate block shown in Figure 1.
It says, "P# block of 4, 1#+B" — plate number block of four, one number plus B.
When more than one design is used for a specific issue, the plate block may be larger than four stamps.
An example is the 32¢ Flowering Trees issue of 1998, shown in Figure 2. Five different designs were issued in a single pane of 20 stamps.
To show each design in a block configuration (two or more rows), the catalog describes the plate block for this issue as "P# block of 10, 2 sets of B+6#."
That means 10 stamps are needed for the plate block, and the attached margin paper will show two sets of plate numbers. The plate numbers consist of the letter B followed by six digits.
The plate numbers described in the catalog listing can be seen at the bottom left and right of the block shown in Figure 3.
Because each stamp design must be represented in the plate block, there are times when the plate block collector saves the entire pane.
One example is the 20-stamp Insects & Spiders issue that was released Oct. 1. Each stamp in the issue is different, so a plate block consists of the entire pane of 20 stamps.
Plate numbers can also be found on many U.S. booklet and coil issues.
Booklet panes have often showed plate numbers on margin paper attached along the top.
This was true when most stamps were perforated (separated from one another with rows of little holes) and used water-activated (lick-and-stick) adhesive.
Modern self-adhesive booklet stamps may appear different from those older issues, but the plate number still appears regularly on most booklets.
Figure 4 shows two different ways that plate numbers appear on modern self-adhesive booklets.
At left on the 32¢ Flag Over Porch booklet of 20 stamps from 1995, the number is printed on selvage above the stamps.
At right on the 32¢ Yellow Rose booklet of 30 stamps from 1996, the number is printed directly on the lower-left stamp. Only one stamp from each booklet includes the plate number information.
Many other issues in self-adhesive panes include plate numbers on peel-off strips that are found in between rows of stamps on the pane.
Most collectors save the entire pane in such cases. The Scott catalog does not list plate blocks for booklet panes or self-adhesive panes with peel-off strips.
Booklets and coils are also printed in large sheets that are simply cut apart differently to fit in the correct format.
For example, coil stamps are printed as large, wide rolls that are slit into strips. Each long strip is then wound into coils of various lengths.
For many years plate numbers were printed on these large rolls as well, but the numbers were trimmed away before the coils of stamps were processed.
Once in a while a collector would encounter a partial plate number on a coil stamp if the coil stamps were cut off-center (high or low), exposing part of the number that was supposed to be trimmed away.
When plate number policies changed in 1980, stamp printers began adding small plate numbers at regular intervals directly on coil stamps. Some collectors began saving these plate number coils in strips with the plate number appearing on a stamp in the center.
One popular format is the strip of five stamps, as shown with the 23¢ Lunch Wagon coil from 1991 in Figure 5.
Because more than one plate might be used for a single issue, collectors keep track of the different numbers that are discovered in post offices around the country.
Those numbers are eventually recorded in catalogs such as the Scott U.S. specialized catalog and the Durland Standard Plate Number Catalog published by the Bureau Issues Association.
Retail stamp dealers also keep track of different number combinations that are discovered and provide the information for their customers in newsletters and price listings.
Sometimes collectors discover that a certain number combination is scarce, and it may become a premium item.
For that reason, a strip of five 18¢ Flag stamps from 1981 including a stamp with plate No. 6 has a Scott catalog value of $2,750, while a nearly identical strip including a stamp with the common plate No. 5 is valued at only $5.
Many collectors also enjoy finding postally used examples of stamps bearing numbers, as shown in Figure 6. The 33¢ Blackberry stamp at left is a plate number coil single, while the mailer who used the John & William Bartram stamp at right also affixed the stamp margin paper with plate information.
Not every issued stamp shows a plate number somewhere on the stamp or in the selvage.
Some recent examples of stamps without plate numbers are the 33¢ American Glass issue released June 29 and the 33¢ Sonoran Desert issue released April 6.
Panes of 10 of the 33¢ Daffy Duck stamp issued April 16 also did not bear plate numbers, but collectors who ordered a press sheet bottom-half by mail from the Postal Service's Stamp Fulfillment Services division found that plate numbers appeared along the sheet margin.
Plate numbers have been a part of U.S. stamp collecting for almost as long as U.S. stamps have existed.
Plate number collecting is just one option available to the collector of U.S. stamps.