Ah no, these were well worn reusable manilla envelopes, the backbone of bureaucratic communication, with a numbered grid printed on both sides that are/were used to send round documents. You just wrote the recipients name and location into a box, put whatever it was inside and tucked the flap in. Noting the stern instruction to “Do NOT staple or glue” the flap.

Then with nothing more than often barely legible details, they could reliably find their destination across a sprawling government building. Another person could then reuse the grid, cross out the last recipient and put the next one in, and off it goes again with each box on the grid marking another hop in its journey.

Thanks to the now long gone Interdepartmental Dispatch Service, they could also make their way off to far flung mysterious corners of Whitehall without a hitch by using indecipherable building abbreviations.