The vent would be a drain. Correct if that was 3 deck it might have been a pantry drain. Depending on where the cabin was. A/c drains went into the general scupper system until later on in life when the a/c drains were used for laundry water.
"There is really only one riveted strip on the QE2 body, where the steel hull meets the aluminium superstructure."
I beg to differ.
I think you will find that the bilge keels were rivetted to the hull.