Resources that may help you date an undated map
Bartholomew (pdf). See also the Old Maps of Essex website.
Date Codes for Maps. From the Western Association of Map Libraries website. Mainly American publishers but includes Geographia and Geographers A-Z maps.
Dating a Map. From Map Curators' Toolbox on the British Cartographic Society's website. Including Geographia Limited and Bartholomew maps.
How Old is Your Globe? From Replogle Globes. A list of name-changes for countries with dates.
Johnson U.S. Map Project. Dataset of the changes in Johnson's U.S. and state maps which will allows the user to definitively date the publication of any loose map from these atlases.
List of world map changes. A Wikipedia page so also use other sources!
Michelin Maps. This site also has sections on covers and advertisements.
Old Maps of Essex. Lots of useful information.
Petrol Maps. The history of oil company road maps in Europe. Includes section on dating oil company road maps.
Rand McNally and Gousha maps. On the website of the Road Map Collectors Association. More detailed information is available to members only.
World Maps. Flow-Chart to determine age of undated world maps. Useful and fun! As a Word file