Thanks for the reply.
Gun access makes sense, but there is no easy method of removal. as there is an inner and outer circle of holes through both inside and outside metal rings. There are 11 holes on the inner row (these have attached flanged nuts riveted to the inside metal ring), and more , maybe 18 holes on the outer row. These have to be attached with screws and loose nuts. Unless there is something amiss with our setup, that would mean that a whole bunch of screws need to be removed to gain access. If the ports were used for loading of ammo and removal of casings, this setup doesn't lend itself to the rapid-turnaround required in wartime.
We're a bit puzzled.
Update: I've seen in photos that there should apparently be only the inside 11 holes. That would make sense . If the inside metal ring was held in bu screws through the plexi into the inside ring, then the panel could be removed bu removing the inside circle of 11 screws. This would remove the panel and outer ring, and leave the inside ring attached to the turret.
Maybe all has been revealed!?