Zygalski sheets
Some hundred and fifty perforated paper sheets stacked on a light table until light came through in exactly one place — which was Enigma's rotor setting.
Henryk Zygalski devised the method in 1938 at the Biuro Szyfrów, after the Germans stopped using the fixed ground setting the bomba relied on. The remaining weakness was the females: because the message key was still sent twice, the same letter sometimes appeared at positions 1 and 4, 2 and 5, or 3 and 6, and that can only happen for certain rotor settings.
For each rotor order, 26 sheets were made, one per position of the left rotor, ruled as a 51×51 grid covering the settings of the other two. Wherever a female was possible, a hole was cut — around a thousand holes per sheet, sliced by hand with razor blades. The sheets for the observed females were laid on top of each other on an illuminated glass plate and offset according to the indicators. If a single aperture remained, the rotor setting had been found.
In December 1938 rotors IV and V were introduced. Sixty rotor orders needed 1,560 sheets, and Poland could neither cut nor fund them. Bletchley Park produced a complete set under John Jeffreys, drove it out to the Poles in France, and on 17 January 1940 Enigma was read again — with British paper and a Polish method.