The Letter codes appear on the back cover of a particular item, bottom left corner. The Letter codes correspond to the year printed. Here are some examples of literature codes and how to read them:

From a 1970s Pickup Truck Brochure: the number appears: AD-45556-B16 Looking at the chart below, "B" dates this as 1973

From a 1980s Tractor Brochure: the number appears: AD-35130-N Looking at the chart below, "N" dates this as 1984

From a 1930s Truck Brochure: the number appears: A-283-CC Looking at the chart below, "CC" dates this as 1939

From a 1940s Refrigerator Brochure: the number appears: A-3-LL Looking at the chart Below, the "LL" dates this as 1948

From a 1960s Farm Equipment Brocure: the number appears CR-1036-M1 Looking at the chart Below, the "M" dates this as 1962

Note: Sometimes but rare the brochure may have been printed the year prior to the actual model year of the product, for example:
From a 1979 Scout Traveler Brochure: the number appears: AD-40589-G2 Looking at the chart below, "G" dates this as 1978, but remember it was printed in late 1978 for a 1979 model.

UNITED STATES
Letter Code and Year
A - 1911
B - 1912
C - 1913
D - 1914
E - 1915
F - 1916
G - 1917
H - 1918
I - 1919
J - 1920
K - 1921
L - 1922
M - 1923
N - 1924
O - 1925
P - 1926
Q - 1927
R - 1928
S - 1929
T - 1930
U - 1931
V - 1932
W - 1933
X - 1934
Y - 1935
Z - 1936
__________________
AA - 1937
BB - 1938
CC - 1939
DD - 1940
EE - 1941
FF - 1942
GG - 1943
HH - 1944
II - 1945
JJ - 1946
KK - 1947
LL - 1948
MM - 1949
NN - 1950
_________________
A - 1951
B - 1952
C - 1953
D - 1954
E - 1955
F - 1956
G - 1957
H - 1958
I - 1959
K - 1960
L - 1961
M - 1962
N - 1963
P - 1964
R - 1965
S - 1966
T - 1967
U - 1968
V - 1969
W - 1970
X-Z - 1971
________________
A - 1972
B - 1973
B/C - 1974
C - 1975
D - 1976
E/F - 1977
G - 1978
H - 1979
J - 1980
K - 1981
L - 1982
M - 1983
N - 1984
__________________
CANADA
Letter Code and Year
A - 1946
B - 1947
C - 1948
D - 1949
E - 1950
F - 1951
G - 1952
H - 1953
I - 1954
J - 1955
K - 1956
L - 1957
M - 1958
N - 1959
O - 1960
P - 1961
Q - 1962
R - 1963
S - 1964
T - 1965
U - 1966
V - 1967
W - 1968
X - 1969
Y - 1970
Z - 1971
A - 1972
B - 1973
C - 1974
D - 1975