On the Subject of 501
What's so special about 501, you ask? Well, I don't know either!
See Appendix BLANK in Blind Alley for blank modules identification.
This module will initially appear blank. Upon your first hold of this module, a tone will play, and one second later, the module will start cycling through 10 3-digit numbers. (For convenience, the first stage will always generate ten 000s.) Do NOT release the module immediately, as this will cause a strike.
Every other cycle will immediately start showing the numbers.
If another non-ignored module is solved, the cycling numbers will change.
To solve the module, release when the screen displays 501. Releasing on any other number or skipping a stage with a 501 will cause a strike. (If every non-ignored module is solved and this module is not yet solved, every number becomes valid. Release at any time to solve.)
If you are holding the module, you may release during the pause to avoid a strike.