While we are on the subject, that's some drivers hatch.
You can see this in Norts top color pic:
"The Birdcage gun sight
In front of the commander’s cupola was a strange looking contraption that looked like a small birdcage but without the wire mesh fitted. It was given the nickname ‘the birdcage’ but was a distant target blade-vane gun sight. It was used by the commander to line up the turret on the target. With the hatches in the locked down position, the commander had 360-degree vision in his rotating cupola."
"Notice the Comet tank commander’s manual gun-laying blade-vane gun sight in front of his open hatch known as the ‘birdcage’
– Photo: commons.wikimedia.org"