I pulled the following explanation from
http://www.critesspeakers.com. He is a well known Klipsch expert:
KLIPSCH S/N FORMATS
DATESDESCRIPTIONEXAMPLE
1946-1947###001 (ending #021)
1948-1961†####0121 (starting #0121
1962-1983##letter####20Y1234
1984-1989YY WW ####89281234
1990-1997DOY Y2Y1 ####135791234
1998-2000YY WW ####00281234
1962-1983 Letter format
A = 1962 F = 1967 K = 1972 R = 1977 X = 1982
B = 1963 G = 1968 L = 1973 S = 1978 Y = 1983
C = 1964 H = 1969 M = 1974 T = 1979
D = 1965 I = 1970 N = 1975 U = 1980
E = 1966 J = 1971 P = 1976 W = 1981
NOTES
1946-1961 dates can only be found in log book (eng. Library)
DOY = day of the year
YY = year (i.e. 99,00…

Y2Y1 = 2nd digit of year, 1st digit of year
WW = week of the year
Date code on drivers (1994) Example - 9429 = 1994, 29th week of the year / YYWW
† Klipschorns of this vintage had the s/n hammer stamped into the tailboard, woofer access door or inside
the woofer chamber.