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.
The form for the entire code varies as well: sometimes the two-letter code is the first element, sometimes a numeral precedes it.
The codes are generally publication dates jumbled in a certain, consistent way that are often preceded by the letter C, although this is not always the case.One final note: Diversified's use of its code was spotty, at best. Therefore, the presence of this code on a Diversified map will allow you to date the map.However, the absense of the code does not mean the map was necessarily issued in one of the years before Diversified began using its code.Rand Mc Nally USGS Topographic Maps A number which often appears on the panel or in the margin of a map.Take the last two digits of the last four numbers and invert them, e.g., 3680 1209R = 1990.