GPO Telephones - House Exchange System No.3

House Exchange System No.3

All 700 series telephone instruments came with at least one knockout forward of the switchooks, for installing bell on/bell off switches for instance. Some, however, the Tele. 710, came with four smaller knockouts and, if you looked inside the case, two circular ones too, top right and left of the dial aperture. These, along with a host of unused holes and bracketry, on the chassis, were a source of mystery for a time. Until, that is, the small business successor to the 300 series, House Exchange System 1 was revealed. The 700 series, HES 3.

This comprised, up to, five stations, all of which could call one another and make, answer or transfer calls. All of the parts of this system had to be built up, on site, by a High Grade Fitter, from an assortment of anonymous looking cardboard boxes and plastic bags. After installing the fixed wiring and external power supply, up to, five ordinary looking 710Ls had to be transformed into Extension Stations by removing all the knockouts and stuffing them with an incredible amount of springs, wiring and lights. Incredibly, two of the knockouts could wind up, for a full system, containing two pushbuttons each, one for each of the other four stations, L shaped to cram them into the small holes. That made a total of two supervisory lamps and six buttons on what, outwardly, seemed a normal, domestic telephone instrument. Getting the case back on took some doing.

• Probably the ultimate development of the 700 series telephones. A HES 3(House Exchange System) built into the shell of a 710 telephone.

• Showing the inside of an HES 3 instrument, crammed with parts