I see you have posted one of "my"pics above of JB910 (chaddy's content). That photograph was taken in late May 1943 after the mid upper turret had been (re)fitted. I know for a fact that the mid upper turret had been removed (or not fitted) in early May 1943. Note that just near the crew entry door there was also a perspex bister which was a mod, and used by the 'redundant' mid upper gunner as an observation blister for 'below and behind'. No gun was mounted in that position on JB910. This same mod can clearly be seen on the aircraft of other squadrons of the time.