Complex sentence: That fourth rotor was one of two types, "Beta" or "Gamma", and never stepped, but it could be manually set to any of its 26 possible positions, one of which made the machine perform identically to the three-rotor machine.
Split into simpler sentences: That fourth rotor was one of two types, "Beta" or "Gamma", and never stepped, but could be manually set to any of 26 positions. One of the 26 made the machine perform identically to the three-rotor machine.