Jefferis started working on sabotage devices for the "Military Intelligence Research" (MIR). When MIR was combined with other hush-hush elements to form the
SOE, Jefferis' unit was not included and it instead became a department in the Ministry of Defence; the only unit of the Minister of Defence (The Prime Minister,
Winston Churchill) and was known as "
MD1", ultimately based in a house called "The Firs" in Whitchurch near
Aylesbury in Buckinghamshire England.
The unit was responsible for the design, development and production of a number of unique special forces and regular munitions during the Second World War. It gained the nickname "Winston Churchill's toyshop".
Jefferis was an explosives expert and engineer, but lacked the ability to manage men well. He was assisted in the management of MD1 by a wily assistant – Major
Stuart Macrae, whose book Winston Churchill's Toyshop, is still one of the few published works on this unique unit.
Over the period of the Second World War, MD1 was responsible for the introduction into service of a total of 26 different devices. Their designs include the
PIAT, the
Sticky bomb and one of the first
magnetic Limpet naval mines.
Through the application of the Squash head and HEAT technology they had a role in the development and production of Lt-Col
Stewart Blacker's
Blacker Bombard, the PIAT (Blacker's smaller version of the bombard) matched to a hollow charge warhead,
Hedgehog (effectively an adaption of the Bombard spigot mortar principle working with the Navy's
Directorate of Miscellaneous Weapons Development) and tank variants including the
AVRE with its "Flying Dustbin" 230mm Petard
spigot mortar, and a bridge-laying tank.
Jefferis developed the idea of the squash head further. His most ambitious project was a bomb designed to sink
capital ships, his ideas were put forward by himself and
Lord Cherwell in 1944 and coincided with the Admiralty's interest in developing a homing bomb for use against the Japanese. The development of this weapon was supported by the Air Staff and MAP who allocated it a higher priority that any other anti-capital ship weapon. When the war ended, development of the 'Cherwell-Jefferis' bomb was continued under the code names Journey's End and
Blue Boar.
Jefferis' development of the hollow charge led ultimately to the same design being used, after refinement by
James Chadwick, in the core of the plutonium bomb dropped on Nagasaki.
