Midland and Scottish Air Ferries
The airline believed that as long as they did not use special stamps they would not be breaking Post Office rules by operating an air mail service. They provided an air service for the 1934 British Industries Fair in Birmingham and flew mail between Castle Bromwich, Birmingham and Heston in London and between Castle Bromwich, Birmingham and Liverpool for two days, before the Post Office stopped them. A fee of 3d was charged in addition to the normal postage, and the mail was posted on arrival. Different cachets were used at each airport.
19th-20th February 1934. British Industries Fair.
No Value
Airport cachet.

Midland and Scottish Air Ferries. Air Mail Magazine Volume 1, pp. 52-53, A. Phillips (1939)
The Air Mails of the British Isles Chapter 7, H. Stanley Redgrove, Privately Printed, (1940)
The Story of the Airway Letter Service 1933-1988 D. Wilson, Air Mail News, June 1994