I have just reviewed an old video purchased by me in Canada in the mid-eighties. It is called "Nightbombers" and claims to be the only colour footage of Lancasters actually filmed during the war. It was filmed and directed in 1943/4 by Air Cdre H.I. Cozens, CB,AFC and shows operations from RAF Hemswell of Nos 150 and 170 Squadrons No 1 Group Bomber Command. At the beginning of the film there is footage of maintenance on Lancasters with very detailed coverage of the actual installation of twin 50 machine guns in the tail. It actually shows the complete turret being taken out by a vehicle mounted crane to the aircraft and placed in position.