Source: https://www.flickr.com.Uploaded to Flickr by Mikey Ashworth and tagged with "kabel". License: All Rights Reserved.

The GPO issued a series of 'Green Papers" during the 1930s – "primarily for the information of staff…" and "to place at the disposal of its officers up-to-date and accurate information…". They were also 'forwarded post free to any interested member of the public". This, the 24th in the series (the 'A' stood for 'postal and general') was about the work of the Travelling Post Offices" that ran on the UK's main railway lines.

The covers were re-designed at about this date (1936) to use this striking design instead of an earlier very bland cover. Sadly no artist or designer is given.