This box would have been used by Mrs. Thatcher whilst serving as Parliamentary Secretary to the Minister for Pensions and National Insurance. Mrs. Thatcher held this position from 9 October 1961, during which time John Boyd-Carpenter, Niall Macpherson and Richard Wood held the office of Minister for Pensions and National Insurance. Her tenure in the department ended on 15 October 1964 when the Conservatives lost the election and a Labour government was formed.

The black box is a travel version of the red ministerial box being less conspicuous than the red. While information classified as ‘Confidential’ or below can be transported in an ordinary lockable briefcase for higher security level documentation with classifications such as ‘Secret’ ministers use ministerial boxes, which are more secure. A ministerial black box with an additional red stripe known as ‘old stripy’ is a highly secret box of confidential papers only seen by the Prime Minister, the Private Secretary and intelligence officials.