Now is a great time to become a day telephonist. The pay is good and the work is satisfying, as well as responsible. It's a job filled with variety and interest - STD handles all the routine calls - and you'll be working with friendly people in pleasant surroundings.

Scan: Leaflet - Post Office Telecommunications PA 99 (2/78) Printed in England by Uniprint London Ltd.

Red marker no.11 Telecom Instruction D2 E3112 of 'The Operating Handbook' says...

BBC Recording Calls Place marker No.11 over speak key, or tie label on answering cord, (cord-switchboards) to guard against entering circuit unnecessary or for premature disconnexion. This is essential to avoid including on recording, operating or switchroom noises.

Quite simply it's a sleeve placed over the speak/monitor key to prevent it being operated in error on important calls such as voice recording or data.