On January 7, 1961, a Canadian couple named Helen and Peter Kroger, owners of an antique bookshop in London, were arrested by British Secret Services at their bungalow in Ruislip, a sleepy suburb on the northwest edge of the city. They were part of Cold War Britain’s most infamous spy ring, passing secrets about the Royal Navy’s latest underwater weapons to the Soviets in Moscow. A search of the Kroger’s residence found encrypted documents hidden in a cigarette lighter; a transmitter that could send one-second blasts of Morse code; and, in typed correspondence, microdots of tiny text disguised as punctuation marks—periods!—readable only under a microscope.

These clandestine artifacts, on display in a recreation of the Kroger home that’s part of a new exhibition at London’s Science Museum, have never been revealed to the public before. “As far as I know,” says Dr. Elizabeth Bruton, the curator of “Top Secret: From Ciphers to Cyber Security,” “we’re the first U.K. museum to borrow them from MI5.”