1st 2nd 3rd base dating
For example, a street map of Denver (OCLC #27850962) bears the statement "1992 edition," but also has a 1991 copyright date, so the two-letter date code for 1991 is used.
The codes for the last three years are "reversals" of the codes for 1981-1983.
Do not confuse with the stock number, which is also in the same vicinity.
(Source: David Cobb, Harvard Map Collection)Based on an e-mail exchange between Louise Ratliff and Morgan Yates.
C - 1 U - 2 M - 3 B - 4 E - 5 R - 6 L - 7 A - 8 N - 9 D - 0 The website for the Road Map Collectors Association (linked above), contains a list of Date Codes used by Rand Mc Nally and H. During much of this period, an explicit statement of date appears somewhere in the map, such as in the legend.
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.
The form for the entire code varies as well: sometimes the two-letter code is the first element, sometimes a numeral precedes it.
For example, on a sheet with separate maps of Arizona and New Mexico, the codes read "11-AN-588-J" and "AN-2-598-J," respectively.