A new design, for a new material. One of the first telephones in the UK made of thermoplastic, in this case polystyrene, rather than bakelite. It was launched in 1958 and used in private installations and not by GPO.
It is nicknamed the horseshoe, because of the horseshoe shaped indentation along the dial. That indentation was designed for putting on lettering, that is combined with the numbers in certain countries. In practice lettering was hardly ever put on. It was made in a wide range of colours and colour combinations and was exported all over the world, mainly to commonwealth countries.