Columbia subject headings:
Harvard subject headings:
Great Britain. Royal Navy
Petard (Ship)
World War, 1939-1945--Cryptography.
World War, 1939-1945--Naval operations, British.
Princeton subject headings:
Great Britain. Royal Navy
Petard (Ship)
World War, 1939-1945--Cryptography.
World War, 1939-1945--Naval operations, British.