[1] By 1936, the membership of the federation was described as "surveyors, under-managers, overmen, master wastemen, electricians and engineers, surface foremen, screening plant foremen, and clerks".
About three-quarters of its members were based in Durham and Northumberland, with the third-largest district being Lancashire, which housed its office, in Bolton.
[3] By the 1940s, the federation was known as the National Federation of Colliery Officials and Staffs, reflecting its broader membership, and in 1945 it affiliated to the Trades Union Congress (TUC), with a membership of 8,183, of whom 120 were women.
Members of the federation voted heavily in favour of joining the NUM, and did so on 1 January 1947, becoming the Colliery Officials and Staffs Area (COSA).
Two smaller, independent unions of colliery officials voted against joining the NUM.