Dating road maps
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.
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.The exception is the Ordnance Survey, who always dated their maps as far as I have seen, sometimes with a plethora of revision dates, so they are always easy to date (unless someone has cropped off the margins too closely). 1841 15 North Bridge as well 1842-57 22 Nidry Street & 38 North Bridge, Edinburgh 1857-78 6 & 13 George Street, Edinburgh . For the remaining publishers who omitted dates, sometimes they did include a date code that can be translated into a year, or at other times all you have is their address to compare against their known address dates. This section is not specific to Essex maps, but rather applies to all UK antique maps. From 1923 to sometime in 1925 it was Geographia (1923) Ltd. Whilst most county maps were included within Atlases that had dates on them - on their frontispieces if not on every map - maps published as individual counties or areas within covers frequently have no obvious date on them. Founded in 1911, it has had at least 5 addresses: 1911 .