OK, here it is.
The first production Lancaster was L7527, flown on 31 October 1941 with 955-kW (1,280-hp) Merlin XX engines in place of the 854-kW (1,145-hp) Merlin Xs used previously, but somewhat slower because the gross weight had grown from 22680 to 27216 kg (50,000 to 60,000 lb). The mid-upper turret had a different appearance because of a surrounding aerodynamic fairing incorporating a 'taboo track' along which rode rollers which kept the twin guns at high angles when fore and aft to avoid firing at parts of the aircraft. Magazines in the mid-fuselage housed 2,000 rounds for this turret and 10,000 for the new Frazer-Nash FN.30 rear turret, less rounded than the FN.20 of the Manchester (almost the same as that of the Whitley V). Early Lancasters also retained the Manchester's FN.21A two-gun ventral turret, but this was rarely used and soon discarded. (Who was to know that in 1943-5, in the Lancaster's hour of need when dozens were being shot down each night by Junkers Ju-88s and Messerschmitt Bf-110Gs, that this would be the only turret that could have done any good?)
Not much use, since only early Lancs had them (and they were .303s), but some Lancs did have ventral guns.
Sisu
