From
www.kotfsc.com:
The ventral postion was soon deleted on most RAF Lancasters as it was thought unnessesary and took the same position as the H2S radome. Where possible, and unofficially, many crews installed a single 7.7 mm (0.303 in) or 12.7 mm (0.50 in) Browning machine-gun on aircraft lacking the ventral turret in order to deal with the ever increasing 'behind and below' attacks of German night fighters using Schräge Musik, which interesting, did not use tracer ammunition. These were hastily installed configurations usually consisting of the gunner sitting on a bicycle type seat with the ammunition box being bolted to the floor and the gun mounted in a hole cut into the floor. The British would eventually re-introduce the F.N.64 turret on aircraft equipped with G-H radar (an improved version of Gee) since that type of radar did not have the large radome as the H2S required. During 1943/1944 when the use of Schräge Musik on german Nachtjagd (night fighters) became widespread, the new twin-gun F.N.64 power-operated turrets became the most important gun position on the bomber.
F.N.64 turret:
