The BOS business archive is held at the Wellcome Library. The archive contains the minutes of the Council and of ordinary and general meetings, 1907-1985; country meetings programmes, 1961-1975; Transactions of the British Society for the Study of Orthodontics, 1908-1971; a few committee reports, 1920s, 1940s; and two histories of the Society reprinted from the British Dental Journal, published 1968 and 1981.
The Wellcome Library catalogue entry is accessible here