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Map Codes
It is important to understand "Map codes" to be able to quickly date maps published by Rand McNally. 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. Sometimes when they issued a new map, they only updated the State map and not the smaller maps leaving date codes indicating different years on the same map. I think the best example of a map series using Rand McNally map codes is the SOHIO Ohio map series. SOHIO used Rand McNally maps from 1924-1926, 1930-1938 and 1949-1985. See the SOHIO-Ohio-Spreadsheet to see the code sequences Rand McNally used in dating their maps.
From about 1918 to 1926 most of the Maps Rand McNally produced were what I call "hard-cover" maps, the map is glued into a folder. All of these so called "hard-cover" maps I have seen can be dated with a year date that is fairly easy to find either on the map cover or on the map itself. It is interesting to note that in 1927 and 1928 I have not found any maps produced by Rand McNally for Oil Companies, it does not mean they do not exist, I have just not seen one. The prewar map codes on the maps always started with a letter until 1945, making it easy to figure out the year of the map, and after 1945 Rand McNally droped the Letter Codes and started using numbers.
Letter Codes (Sohio starts in 1930 with L)
A 1919 J 1928 S 1937
B 1920 K 1929 T 1938
C 1921 L 1930 U 1939
D 1922 M 1931 V 1940
E 1923 N 1932 W 1941
F 1924 O 1933 X 1942
G 1925 P 1934 Y 1943
H 1926 Q 1935 Z 1944
I 1927 R 1936
Base Codes for SOHIO Ohio maps
1063 1930-1938
4903 1949-1954
554903 1955-1966
6714 1967-1985
The base code identifies a map format created by Rand McNally. When you look at the SOHIO Ohio maps from 1930 to 1938, you will see that each map code contains the number 1063, and is preceeded by a letter. The 1063 is a base code refering to the map format used for those years by Rand McNally, and the Letter preceeding the base code indicated the year the map was issued. From 1939 to 1948 General Drafting was the map maker for SOHIO, then in 1949 SOHIO switched back to Rand McNally and started with the new base code of 4903. The first two letters of the base code represent the last two digits of the date the new base code was created.
Now lets look at the 1951 maps as an example, there are two versions of this map, a 1-4903-4 and a 1-4903-5. the (1) before the base code is the last digit of the year the map was produced, and the 4&5 after the base code are the two revisions of the map made in 1951. The date is always represented by the numbers in front of the base code and the revision number always follows the base code.
Next lets look at the 1967 base code of 6714, the 67 again represents the last two digits of the year the map format was created. In 1970 there are two version of the map, a 706714-7 and a 706714-8, the 70 before the base code is the last two digits of the year the map was produced, and the 7&8 after the base code are the two revisions for the map made in 1970.
Confusion sometimes arises deciding if a map is say a 1955 map or a 1965 map, as both maps would start with 5-xxxx-x. The Sohio 1955 map code is 5-554903V-1 and the 1965 map code is 5-554903V-21. But if one looks at the date of the map base code and assumes say 2 revisions per year it usually becomes obvious how to date the map.
Dating 1956 and 1957 Rand McNally Maps can be difficult, as many of the 1957 maps have 1956 map codes, this is true for many states and many companies. Just note that there is a problem dating these maps, you can refer to individual company spread sheets to see examples of dating for those years.
Other Dates & Codes
There are other dates and codes on the maps as you will see on the SOHIO-Ohio-Spreadsheet, the "Small Map Codes" on the map covers start in 1949 on the front of the map and then move to the bottom of the back of the map. This makes it easy to date these maps without opening them up.
29 A&B represent 1949 thru 40 A&B represent 1960
61 A&B represent 1961 thru 85 A represents 1985
If you are a map seller, I recommend that you include both the "Map Code" and the cover "Small Map Codes" in the auction, so that any knowledgeable collector will know exactly what you have. If you state a date for the map, the potential buyer does not know how you dated the map and will wonder if you did it properly.