The drum cupola was removable. It was bolted into in a base ring that was welded into the turret roof plate. This drawing shows the base ring. The cupola was held by ten bolts through the flanges.
The ring had a maximum diameter of 880mm. The internal flanges were at the level of the roof plate. The hole in the roof was 850mm diameter, but the inside of the ring was only 695mm wide.
This photo shows the base of the cupola from inside. [A] is the edge of the 850mm hole in the roof plate. The base ring [B] sits in it. The cupola armour ring [D] is bolted on top.
This diagram shows the turret roof from below. There was not adequate room in the circular turret for this large cupola. It was placed as far back as possible, overlapping the armour wall and a roof beam. But even so, it was 50mm to the left of the commander's seat.
[1] Survey of vehicle 250122, Bovington, by David Byrden