1939 ... (Don't know if there was one.)
1940 through 1945 ... B (Maybe) <img src="/ubbthreads/images/graemlins/help.gif" alt="" /> <img src="/ubbthreads/images/graemlins/confused.gif" alt="" />
1946 ... C
1947 ... D
1948 ... E
1948 ... F
1950 ... G
1951 ... H
1952 ... J
1953 ... K
1954 ... L
1955 ... M
1956 ... N
1957 ... P
1958 ... R
1959 ... S
1960 ... T
1961 ... U
1962 ... V
1963 ... W
1964 ... Y and Z
1965 ... AA
1966 ... AB
1967 ... AC
1968 ... AD
1969 ... 69
1970 ... 70
1971 ... 71
1972 ... 72
1973 ... 27
1974 ... 26
1975 ... 25
1976 ... 24
1977 ... 23
1978 ... 22
1979 ... 21
1980 ... 20
1981 ... 19
1982 ... 18
1983 ... 17
1984 ... 16
1985 ... 15
1986 ... 14 (Just a guess.)
1987 ... 13 (And another.)
(Notice that, beginning with 1973, if you subtract the prefix number from 100, the remainder equals the last two digits of the year.)