I found my information in the "Blue Book of Modern Black Powder Values".
They start in 1954 with X (10) and proceed onward to 1970 with XXVI (24 for 1970). For instance XVII (17) is 1954 + 7 or 1961.
Then they changed for 3 years to an XX7, XX8, XX9 for 1971-1973.
in 1974 they switched to letters using AA for 1975, AB for 1976 and on skipping some letters like G, J, K, O, Q, R, V, W, X and Y so they look like this:
AA 1975
AB 1976
AC 1977
AD 1978
AE 1979
AF 1980
AH 1981
AI 1982
AL 1983
AM 1984
AN 1985
AP 1986
AS 1987
AT 1988
AU 1989
AZ 1990
Then they went to the B series
BA 1991
BB 1992
BC 1993
BD 1995
BF 1995
BH 1996
BI 1997
BL 1998
BM 1999
BN 2000
BP 2001
BS 2002
BT 2003
BU 2004
BZ 2005
CA 2006
CB 2007
I don't have it written down but I believe 2008 is CC.