Just a thought but would the ventral gun have required a further crew member or would they have used the W/O?
I think the original idea was for the turret to be manned on an as and when basis, like the front turret. The problem with the FN64 being that the gunner had no free view, only what he could see through his periscopic sight. He would have had to rely on help from the other gunners to "acquire" his target.
Those Squadrons that re-installed the turret in 1944 may well have carried an extra gunner, given that a night attack using Scrage Musik could come at any time and without warning.
Also bear in mind that the turret would be useless in the event of an attack if the oil had not been regularly circulated to keep it warm. That requires someone to operate the controls and exercise the turret quite vigorously for several sessions per hour.
So, our poor Air Gunner would have had to sit in the freezing temperatures of the fuselage for hours at a time, squinting through the periscopic sight into a very limited field of vision in the hope of spotting that shadow before it got into a firing position.
The turret traversed 100 degress either side of the centreline and elevated to +2 degrees and -67.5 degrees from the horizontal.
As an extra comment, I read yesterday that there were
official trials to install a fixed .5 in gun into Bomber Command aircraft, specifically to counter the Schrage Musik attack. This is mentioned in Sir Arthur Harris' Dispatch On War Operations. It does not state how many operational aircraft actually got them - my guess is very few.