GPO joint marker ‘No4’. Cast iron wall plate marking the position of buried couplings, and buried jointing chambers for telephone cables.
Historic Context: A number of privately owned electrical telegraph companies operated in the UK from 1846 onwards. Among them were: Electric Telegraph Company, the world's first public telegraph company, which developed a nationwide communications network; British & Irish Magnetic Telegraph Company; British Telegraph Company; London District Telegraph Company; and the United Kingdom Telegraph Company. The Telegraph Act 1868 passed the control of all these to the Postal Telegraphs Department of the newly formed General Post Office (GPO). The Telegraph Act 1869 granted the GPO a monopoly over communications. With the invention of the telephone in 1876 the GPO began to provide telephone services from some of its telegraph exchanges. It was confirmed in 1880 that the 1869 Act included telephony even though the telephone had not been invented when the Act was first conceived.
In 1882, the Postmaster-General, Henry Fawcett started to issue licences to operate a telephone service to private businesses. The telephone system grew under the GPO in some areas, and under private ownership in others. The GPO's main competitor, the National Telephone Company (NTC), emerged in this market by absorbing other private telephone companies. The NTC controlled most of the telephony in Britain before the 1880 ruling on the Telegraph Act 1869 mandated a nationalised service. This was instated in 1911 prior to the absorption of the NTC into the GPO in 1912.
A few municipally owned services remained outside of GPO control. These were Kingston upon Hull, Portsmouth, and Guernsey. Hull still retains an independent operator, Kingston Communications, though it is no longer municipally controlled.
References: Information on marker plates from ‘britishtelephones.com’ website (an historical website run by Bob Freshwater about UK customer telephone apparatus and systems).
History of the GPO from Wikipedia.