3rd base dating
In 1933, General Drafting Company was the first company to produce four-color road maps. Thus a map of Baltimore which has the following numbers on it: N132 CR654 was published in June of 1954. Cram Company In or near the border at one corner of each map [atlases are coded differently] is a coded reference to the date of revision.In May, 1952, the company moved from New York City to Convent Station, N. Substitute numbers for the following letters to get the month and year: A - 0 B - 9 C - 8 D - 7 E - 6 F - 5 G - 4 H - 3 I - 2 J - 1 At the bottom left corner of each map (usually) are some letters.Maps from the 1930s will often have three-digit codes containing just the month and last two digits of the year, generally in that order; 336 = March 1936, for example. 15 1957 (date  last two digits of year reversed [75 = 1957] month ) 124511 = Nov. Clason maps are notorious for having no date information.12, 1954 (date  last two digits of year reversed [45 = 1954] month )1744 = 1947 ["74" reversed is "1947"] 218412 = 1948 20353 = 1953 1354 = 1953 C-11558 = 1955 15658 = 1956 C-15857 = 1958 C-1859 = 1958 C-5361 = 1963 After this date they seem to use more standard dating, e.g., 9-67 = 1967. Instead, various elements of the maps changed over time.If the code contains four digits, the first two digits reference the base map.
(I have not been able to find date codes on maps issued by Diversified in 1962 or 63).Therefore, it is not too hard to figure out the year of a map containing the single digit date code..Here's an example: I have a 1964 Standard Oil map of Minnesota produced by Diversified. The 14 references Diversified's base map of Minnesota; the 2 suggests that the map is a second edition for the year of issue; and finally the 4 indicates that the year of issue is 1964.This allows for construction of a list of proper dates.
For earlier decades, Gousha indicated the year by a single letter (A-Z), then by double-letters (AA-ZZ, the last being from 1978).
Rand Mc Nally USGS Topographic Maps A number which often appears on the panel or in the margin of a map.