The cyclometer was used to prepare a catalog of the length and number of cycles in the "characteristics" for all 17,576 positions of the rotors for a given sequence of rotors. Since there were six such possible sequences, the resulting "catalog of characteristics," or "card catalog,", when it was ready in 1937, comprised a total of (6) (17,576) = 105,456 entries. Preparation of the catalog, writes Rejewski, "was laborious and took over a year, but when it was ready... daily keys [could be obtained] within about fifteen minutes."
On November 1, 1937, however, the Germans changed the "reversing drum," or "reflector." This forced the Cipher Bureau to start over again and produce a new card catalog, "a task," writes Rejewski, "which consumed, on account of our greater experience, probably somewhat less than a year's time."
But then, on September 15, 1938, the Germans changed entirely the procedure for enciphering message keys, and as a result the card-catalog method became completely useless.
This spurred the invention of Rejewski's cryptologic bomba and Zygalski's perforated sheets.