I agree that most Mainforce lancs had a upper turret, in fact most all lancs had them, the point I was making was that most of those that did fly without them were not anything special but were just supply shortages and were treated eaxactly like anyother lanc.
I have NEVER found anything that implied that the turrets were removed, all those without were either specials or supply shortages and all had the turret mods done at the factory.
I was a a talk once given by ex staff at the AVRO factory at Yeadon and this point about turrets came up, they do not remember any Lancs leaving yeadon without a full set. (although they also never fitted belly turrets)
an intertersting story from that talk was of a lanc pilot who told a tail of landing after a Berlin raid to be told that he had flown all the way home withing spitting distance of another Lanc and had never seen anything.
ps shooter has only ever hit a true fact by complete accident, and the fact that the B17 needed to replace half its bomb looad with additional fuel tanks to reach ranges the lancs were doing with 10000+ lbs is upto his usual standard.
pps
Someone mentioned the Mossie and berlin raids, seemingly the mossie had a 2 hour flight time to berlin with a 4000lbs cookie (note that note all the bombs that the mossie carried that looked like the 4000lbs cookie were, a proportion were actually incendaries that used the same case)