If any GPO engineers understand the naming (for example, the odd ZDOTR, then please write in). Most of the metal covers remained enigmatic however, just being numbered.

"I might be able to give a little more information on some of the labelling. The room you describe as a telephone exchange looks to be a frame room. The wiring blocks you saw are part of a distribution frame. Distribution frames are where cables that come into a building are connected to equipment or to cables leading elsewhere in the building. The connections are made with pieces of wires known as jumpers."

"ZDOTR could be a site code (known as A1141 codes) showing where a cable is heading to or it could be a piece of equipment - the letters TR often indicated a traffic recorder in electromechanical exchanges. Cables between sites are known by their A1141 codes and then a number (alphabetically superior first) - so for instance the second cable from Brighton to Horsham would be the "BR-HMC No.2."

"MDF stands for Main Distribution Frame - this is the main frame in a building and is where external cables are terminated. Other frames include RDF (used in repeater stations where signals were amplified midway between exchanges) and IDF for Intermediate or Internal - used within exchange floors to physically connect the different parts of the exchange to each other."

"Essential and Non-Essential electrical circuits relate to how the building is wired. If mains power fails non-essential circuits stop working and essential circuits are powered by a battery back-up and a generator. The distinction between the two is because the generators would normally not have enough power to power every circuit in a site and/or the fuel would last longer if only certain items remain powered."

Terry Barnes