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Map Codes
It is important to understand "Map codes" to be able to quickly date maps published by H. M. Gousha. Map codes are in very small numbers and letters and are located on the bottom line of each map. There can be different map codes on each sub-map that is part of the overall map. For this discussion I am going to refer to the map code on the large state map on any given state map. I think the best example of a map series using the Gousha map codes is the CONOCO Colorado map series. CONOCO used Gousha maps continuously from 1927 to 1981, so by looking at the Conoco-Colorado-Spreadsheet page you can see the complete Gousha map code sequence for CONOCO Colorado maps.
Letter Codes
Gousha map codes start in 1927 and include the letter "A" and continue with a single letter each year until 1952 with the letter "Z" then they start over using double letters in 1953 with the letters "AA" and continue using double letters until 1978 with the letters "ZZ", from that point on they get a little confusing and I will not discuss that here.
A 1927 N 1940 AA 1953 NN 1966
B 1928 O 1941 BB 1954 OO 1967
C 1929 P 1942 CC 1955 PP 1968
D 1930 Q 1943 DD 1956 QQ 1969
E 1931 R 1944 EE 1957 RR 1970
F 1932 S 1945 FF 1958 SS 1971
G 1933 T 1946 GG 1959 TT 1972
H 1934 U 1947 HH 1960 UU 1973
I 1935 V 1948 I I 1961 VV 1974
J 1936 W 1949 JJ 1962 WW 1975
K 1937 X 1950 KK 1963 XX 1976
L 1938 Y 1951 LL 1964 YY 1977
M 1939 Z 1952 MM 1965 ZZ 1978
Map Base Codes
As part of the "Map Code" you will see a three digit number that changes every few years, this is the map base code, I assume it was revised each time they made a major revision to the map format, you will see the Colorado map base codes are as follows:
141 from 1928 to 1930
241 from 1931 to 1935
341 from 1936 to 1940
347 from 1941 to 1946
519 from 1947 to 1966
601 from 1967 to 1981
Other state maps have different map base codes but follow a similar pattern.
J or JC
Many of the Gousha "Map Codes" include the letter J or JC. I don't know the meaning of these letters, but they can sometimes confuse dating maps by map code. They don't seem to have any value in dating Gousha maps.
Month Code
Many map codes contain a number between 1 and 12, this appears to be a month code and will vary in location in the codes.
Small Date Codes
"Small Date Codes" are separate from the "Map codes" and in the case of Conoco maps are found on the back cover in the lower right hand corner from 1938 to 1963. They usually contain 4 numbers like 4382, the 4 is the month of issue, the 38 is the last two digits of the year of issue, and the 2 is the revision number. These codes are excellent for quickly dating maps as you do not have to open the map to read the code and for the most part these codes are unique to a given map, but they may not be consistent with the "Map codes" when dating maps.
Other dates
There are other dates on the maps as you will see on the Conoco-Colorado-Spreadsheet including Census dates, Edition dates and copyright dates. You will notice that these dates are often inconsistent with each other and the map codes resulting in inconsistent dating when you are shopping for maps.
Edition Dates are usually consistent with map codes
Copyright dates are fairly consistent
Census dates are the least accurate way of dating maps
If you are a map seller, I recommend that you include both the "Map Code" and the "Small Map Code" in the auction, so that any knowledgeable collector will know exactly what you have. If you just state a date for a map, the potential buyer does not know how you dated the map.