From http://chriskeene.com/kwg-13.htm
Check the Shape Numbers. If the pipe has a 4-digit shape number, the pipe is probably of "pre-1936" vintage. If the pipe has a 4-digit shape number and does not have the threaded fitment, it is probably from before 1925. The information in Chapter 3 on pipe grades, along with the shape lists in the Appendix, can, also be useful in dating a pipe. Because grades and shapes frequently changed, this information can be useful in establishing an approximate date, especially if the pipe in question is of an unusual shape and/or grade. Fortunately, the older, more desirable Kaywoodies (i.e., those of pre-1936 vintage) can be identified by their 4-digit shape numbers.