Time for a quick plug. All of this talk about plate numbers reminds me that you wonít find much of this information in the Scott catalogs, not even the US Specialized catalog.
If you want to learn more about plate numbers and which plates were used to print which stamps, you need to consult a more detailed catalog than the US Specialized. You need the Durland Standard Plate Number Catalog which is available from the United States Stamp Society (USSS). You can order a copy online at the USSS web site (http://www.usstamps.org/books.html). The Durland catalog comes in two formats. Perfect bound is $21 and spiral bound is $23. Non-members can order copies too and they cost $5 more.
If youíre not already a USSS member, I highly recommend membership. The USSS publishes a monthly journal that is full of information about US stamps that you wonít find in the general philatelic press. If you need more information about the USSS and you donít have access to a computer, drop me an SASE and Iíll send you information.