David, all I can say is "well observed"!

I remembered from something I read back in the ancient 70s (maybe the Furious Profile Publication Part 2) that the openings were covered by protective gratings to deflect men or aircraft. Unfortunately, I could not recall the source. It seemed barely credible the RN would risk men or machines plummeting into the trunks, but I had no data, and, oddly, none of the web sources seemed to consider this "little" detail.