Both the Zygalski-sheet method and each "bomba" worked for only a single scrambler rotor order, so six sets of Zygalski sheets and six "bomby" were produced. However, the Germans introduced two new rotors on 15 December 1938, giving a choice of three out of five to assemble in the machines on a given day. This increased the number of possible rotor orders from 6 to 60. The Biuro could then only read the small minority of messages that used neither of the two new rotors. They did not have the resources to produce 54 more "bomby" or 54 sets of Zygalski sheets. Fortunately, however, the fact that the SD network was still using the old method of the same indicator setting for all messages, allowed Rejewski to re-use his previous method of working out the wiring within these rotors. This information was essential for the production of a full set Zygalski sheets which allowed resumption of large-scale decryption in January 1940. On 1 January 1939, the Germans made military Enigma even more difficult to break by increasing the number of plugboard connections from between five and eight, to between seven and ten.