Major 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, 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.
Through the application of the Squash head and HEAT technology they had a role in the development and production of the Blacker Bombard, the PIAT, Hedgehog (effectively an adaption of the Bombard principle working with the Directorate of Miscellaneous Weapons Development) and tank variants including the AVRE with its "Flying Dustbin" 290mm Petard spigot mortar, and a bridge-laying tank.
Prime Minister Churchill became acquainted with then Major Jefferis in 1940 and regarded him as a "singularly capable and forceful man." He recommended a promotion to Lieutenant-Colonel so that Jefferis would have more authority.
He was appointed Knight Commander of the Order of the British Empire (KBE) by Churchill in the 1945 Prime Minister's Resignation Honours.