This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
Estes uses a date code on their rocket motors. It's of the form XXYZZ
(example, 25T9) where the first number is the day of the month of
manufacture, the letter is a code indicating year of manufacture, and the
last number is the month (1 = January, 12 = December). Date codes
run progressively through the alphabet, as follows:
T 1989 U 1990 V 1991 W 1992 X 1993 Y 1994 Z 1995 A 1996 - Estes cycled back to the beginning of the alphabet