As far as collectors are concerned, nothing is special about the letters that sometimes are cast with the pattern number on Griswold pieces and a few other makes. Cast iron pieces are made in a sand MOLD and that mold is made by packing special casting sand around a PATTERN. Each piece of iron has to have its own mold as the mold is a one-time-use thing which is destroyed in order to remove the piece cast in it. In order to make enough molds a foundry might have to have several or more patterns for popular, high production pieces. In this case each pattern has a different letter (plus, usually, one with no letter) on it after the pattern number which would be the same number for like items. It really doesn't make any difference to collectors which pattern was used to make a piece because they were all the same (but sometimes with very minor differences) except for the letter. One of Griswold's most prolific pieces is the No.8 block TM smooth bottom skillet. For these Steve Stephens, a long time collector, has identified all letters of the alphabet, except for the letters E I O U and Y and no letter, and some of these could still turn up. Pieces that were produced in low quantities may have only one pattern to make all the molds that were required. The preceeding deals mostly with Griswold pieces but may apply to other makes as well. Wager called their numbers "catalog numbers" but, basically, they are pretty much the same as Griswold's pattern numbers.