Until relatively recently, maps produced by the Geographers' (A–Z) Map Company did not include a publication date. Some clues to dating include:
On all A–Z maps, there is a 3 or 4 letter code in one of the corners, often the one containing the key. These letters represent numbers, which are the cartographic date, in the form (M)MYY. There was at least one scheme used, and perhaps a second.
In one scheme, the letters JIHGFEDCBA represent the digits 1 to 9 and 0, so that HFD would be 357, indicating a publication date of March 1957. This seems to have been used on all the company's folding maps, and possibly also those in book form.
The Geographers' A–Z Street Atlas and the story of how Phyllis Pearsall came to write the first edition covering London were featured in a 2005 episode of Nicholas Crane's Map Man TV programme. This revealed that, on all their maps, A–Z print a non-existent trap street so that they can tell if a map has been illegally copied from theirs, a technique used by several publishers of reference works (see fictitious entry).
The story of Pearsall's creation of the A–Z also inspired the musical The A–Z of Mrs P.