The carrier toilets that stayed classified

What was the strangest material Dr. Clarke found still classified before it was later released?

Plans for ordinary toilets aboard certain Second World War carriers remained classified even though related catapult and arresting-gear details had been released elsewhere.

Dr. Clarke found carrier toilet plans inside a folder that also concerned aviation equipment. Catapult and arresting-gear material appeared in other released files, yet the toilet drawings remained restricted until he asked about them and they were later declassified with the folder. He could find nothing unusual in the sanitary design itself. The example illustrates how archival classification may follow the status of a file grouping long after the specific content has lost any plausible secrecy.