In the inter-war years and through WW2 there was a GPO Special Investigations Unit in every sorting office.
They were steaming open, photographing mail, then sending them on, and the photos to MI5. They were also responsible for recording phone calls so had presence in major exchanges.
They were steaming open, photographing mail, then sending them on, and the photos to MI5. They were also responsible for recording phone calls so had presence in major exchanges.
https://www.lrb.co.uk/v37/n07/frances-stonorsaunders/stuck-o...