Nice work Andy.
My thoughts are that the two 'boxes' (painted black and grey) are representing the same type of gun sight as fitted to the turrets, the same as the pics of my sight I posted, with the part you've painted grey being the hood over the reflector screen.
It's possible that the sight is folded to a stowed position, and would be vertical when in use, but it is more likely that it is deliberately mounted on its side, in order to reduce parallax effect with alignment with the offset-mounted gun.
The twin hand-grips probably had rubber covers slotted over them, similar to those found on bicycles of the period, more than likely grey or dull red in colour.