Williams components are all marked with a letter or two letters to indicate the season of manufacture which ran from 1st July to 30th June until 1923. The season then ran from 1st August to 31st July thereafter. This list is taken from what appears to be official Williams literature...
                                                                                                        Hilary Stone

191213 A
1914 B
1915 C
1916 D
1917 E
1918/1919 probably E
1920 F
1921 G
1922 H
1923 I
1924 J
1925 K
1926 L
1927 M
1928 N
1929 O
1930 P
1952 AP
1953 AS
1954 AT
1955 AU
1931 S
1932 T
1933 U
1934 W
1935 X
1936 Y
1937 Z
1938 AA
1939 AB
1940 AC
1941 AD
1942 AE
1943 AF
1944 AG
1945 AH
1946 AI
1947 AJ
1948 AK
1949 Al
1950 AM
1951 AN
1956 AW
1957 AX

1958 AY
1959 AZ
1960 ZA
1961 ZB
1962 ZC
1963 ZD
1964 ZE
1965 ZF
1966 ZG
1967 ZH
1968 ZI
1969 ZJ
1970 ZK
1971 ZL
1972 ZM
1973 ZN
1974 ZP
1975 ZS
1976 ZT
1977 ZU
1978 ZW
1979 ZX
1980 ZY