Dating Bartholomew's Maps

I recently picked up an old Bartholomew's map of Merseyside ("dissected and mounted on cloth"). While trying to figure out how old it was, I came across this guide:

http://www.cartography.org.uk/downloads/MCT_BartsMaps.pdf (I've saved a copy in case that link ever dies).

My map clearly has the code "A26" in the top left corner, which from the guide shows that it was printed in 1926.

Has anyone found any similar guides for other old maps?

Swiss topo maps always seem to have the date on the cover of the map, although it's been getting progressively smaller. I have a couple of 1:100k sheets from the 1910s where they give a month and a year on the cover in large, legible type. More recent ones have the year in small type in the bottom corner of the cover (in both cases it's also printed at the bottom of the map itself). The British Ordnance Survey seems to be a lot more coy about dates: they evidently want you to buy the map first, then discover how long it is since they last revised it.