It's May, so must be time for the April Archive of the Month:

"The Post Office Went to War" by Ian Hay, published in 1946 by HMSO. 

“The chapter I have copied to you describes Orkney's contribution to this task. Other chapters in this 94 page booklet cover London, General Post, Military Mail, Wires and Wireless, Engineers and Calais View. If you would like to see the rest of the booklet, please pop by and ask for reference D1/1094.”

Lucy Gibbon, Orkney Library and Archive.