“Coming to the throne in 1952, Queen Elizabeth II has had the longest reign of any British monarch since the invention of the post box. (Queen Victoria reigned for nearly as long, but post boxes only appeared halfway through her reign.) Hence there are more EIIR boxes on the streets of the UK and Commonwealth countries than of any other monarch. Wall box manufacture at the time of her accession was with W.T. Allen & Co. Ltd.
The greatest changes were in the size of the apertures in use. The older six inches (15 cm) wide aperture was not sufficient for modern larger letters and was quickly enlarged – first to eight inches (20 cm) and then to ten inches (25 cm). This, combined with a change in contractor to Carron Co., led to a good variety of boxes in both Type B and the larger Type A. The contract for manufacturing cast-iron wall boxes ceased in 1980, but in the period 1952-1980 more than 20,000 boxes were installed in the UK.
In Scotland there were protests when the boxes made early in the reign of Elizabeth II were produced. These bore the cypher “EIIR” but there were objections because Queen Elizabeth is the first Queen of Scotland and Queen of the United Kingdom to bear that name; Elizabeth I having been Queen of England and Queen of Ireland only. After several EIIR pillar boxes were blown up by improvised explosive devices, the General Post Office (as it was at that time) replaced them with ones which only bore the Crown of Scotland and no royal cypher. Red telephone boxes or kiosks of type K6 were also treated in the same way, so too GPO/Royal Mail lamp and Pillar boxes.”