My father [who is still alive and kicking] was a deck cadet on the John R Park and signed off of her on the trip just previous to her final voyage. Although my father was a cadet, the captian trusted him as navigator more than the 2nd and 3rd mates on the ship. So he spent much of his time on the bridge. Dad's battle station was on the Port side gun tub next to the bridge. I've shown him the photo...he believes this was a floor drain and is unrelated to the 20mm Oerlikon. I've been on both the John Brown and on the Jeremiah O'Brien, but I don't remember any similar item on the AA guns or in the gun tubs themselves.