It's a metal plate, bolted to the fixed cupola ring at the seven o'clock position. The top surface of the plate is mostly flat but there is a chamfer on the side nearest the rotating cupola ring facing the right side. It seems to be related to small rotatable lever fixed to the front of the peri-bino protector. I can't find any mention of it in the gunnery manual section relating to the Vision Cupola No 2.
This is the wonderful answer from an Australian former Cent commander:
“We rotated the cupola clockwise until that device came up against a stop at the rear of the cupola. This placed the peribinos in the 6 o’clock position; why we don’t know, but Tubby is checking all his references for an answer. None of us could remember ever using this facility…….more to follow.”
When the stop is flipped to "engage", it only engages the fixed plate when rotated clockwise (looking from inside the turret). Counter-clockwise, it rides over the plate because of the chamfer. That solves the what but we've been unable to explain the why. When the stop is engaged, the peri-binos are held at the six o'clock position, with the hatch, if open, at eleven o'clock and the RCP sight at the 3 o'clock, none of which is very useful to the commander!
Another case of a design feature which was probably redundant and whose purpose is now lost. Someone obviously thought it useful, although it's not fitted to all Cents.
Rotatable stop:
Stop not engaged with fixed plate:
Stop engaged with fixed plate after rotating cupola clockwise:
Stop over-ridden by fixed plate after rotating cupola counter-clockwise:
Another Cent enigma for the weekend!
Regards
Stephen
