In the inter-war period, the main physical change to the Mount was a new wing, built as a substantial extension to the Letter Office. This extension and refurbishment aimed to bring an unprecedented degree of mechanisation to postal sorting, including conveyor belts that could travel beneath the floors and transport letters between the various stages in the sorting process.
As late as 1917 some of the old prison cells were still being used to store parcels, but by 1929 the last of the remaining sections of the prison were demolished to make way for the extension.
The rationale behind this partial mechanisation was explained in the notes to accompany engineering plans drawn up in 1916: "While there are certain selective processes which can only be carried out by human agents, there is scope for mechanical transportation between processes". Although the costs to mechanise were considerable, there was clear economic incentive behind it. A 1925 estimate stated that, in terms of staff economy, the new system would pay for itself within a single year. This included 12 lifts descending to the Post Office Underground Railway.
On 2 November 1934, the new building was officially opened by the Duke and Duchess of York (the future King George VI and Queen Elizabeth). Over one thousand staff and visitors looked on as the Duchess tried her hand at operating a new stamp-cancelling machine. The ceremony was complete when the Duke operated switches that set in motion the sorting machinery on the ground floor.
The whole of the opening ceremony was broadcast throughout the Mount by loudspeakers erected by the Engineers Department. In a number of speeches made that day, the Duke praised the work of the engineers and architects who had turned Mount Pleasant - once a rubbish heap in an uncultivated field - into what was then the largest postal sorting centre in the world.
Mount Pleasant new wing, 1934
Conveyor belts in Mount Pleasant
Duke and Duchess of York at opening