My father is one of the reasons why Parnall were able to manufacture these.
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To enable the turret to turn and elevate/depress the guns (which gives a very good field of fire for the gunner) the body of the turret "hangs" in a frame. The turning is accomplished with the aid of ball-bearings, held in a "race". These races need to be attached to the support frame, but the nuts and bolts that do the holding must not obstruct the movements.
The problem was that when you make a bolt, the tool cutting the thread must leave the surface of the shaft just before it reaches the head. In the case of the bolts holding the ball-races, there wasn't quite enough thread. The difficulty was more acute because these turrets are actually quite small and lightly built.
What CAS did was to modify the thread on the bolt, by hand, so it went far enough up the shaft for the nut to get a proper grip and stay out of the way of the turret's motion. This was needed because the turret design had been altered - to use thinner and lighter materials - which meant the original design & size of nuts and bolts now didn't fit properly.

Image from my late father's papers