They started stamping the lever boss codes in 1949 but many are very hard to read. They are very small and prone to wearing off over time.
Here is a complete list of the Lever Boss Codes and their corresponding year of manufacture:
A= 1949
B= 1950
C= 1951
D= 1952
E= 1953
F= 1954
G= 1955
H= 1956
I= 1957
J= 1958
K= 1959
L= 1960
M= 1961
N= 1962
O= SKIPPED
P=1963
Q= SKIPPED
R= 1964
S= 1965
T= 1966
U= 1967
V= 1968
W= 1969
X= 1970