"This map has unique record among cartographical publications. The first sheets were brought out in 1875 with simple colouring by counties. At the Paris Exhibition of 1878 Mr Bartholomew showed specimen sheets printed in layer or contour colouring. That system was eventually adopted for the whole publication, which thus became the first topographical series in any country to use it. Based originally on the Ordnance Survey, by permission, it is now kept up to date by its own service of information and is generally acknowledged the most up-to-date of any map in the country".
These maps didn't carry a date, but judging by the text on the cover ("By appointment to the Late King George V"), it has to date from later than 1936, and as it shows the railway line from Bourne End to High Wycombe, it presumably dates from before 1970, when that line closed.
I've also been looking at details of various Bartholomew design changes here. As far as I can make out from the index of sheets it was based on a layout that was used from 1940 onwards, and the cover design seems to date from before 1963. Judging by the original cover price (2/6), it dates from the early 1950's. However, it shows the Maidenhead bypass as being under construction. That became the first section of the M4 in 1963. So perhaps it actually dates from nearer then.
Beneath the map is this note:
"The publishers record their appreciation of the valuable services rendered in th past by map users un assisting to maintain the accuracy of this series and are always pleased to acknowledge any corrections brought to their notice".
I guess it's probably too late to let them know about Beeching, the construction of the M4 and M40, or that there are no longer Youth Hostels in Maidenhead, Henley, Beaconsfield / Chalfont St Peter, and Lane End. Instead (if only there was an agreed tagging scheme) we could all be out there adding abandoned Youth Hostels to OSM.
These days, of course, a fifty-year-old map is of no practical use whatsoever, but it is a thing of beauty nevertheless.