On on Model M standard US ANSI layout, the shift keys and backspace cover two barrels and there's a contact under the unused one that will generate a scancode when pressed, used for other key layouts.
However, the 0, Enter, and + key on the numpad have the same setup, and there's contacts beneath the second barrels here as well, but pressing them does not generate a scancode (on the M I'm currently tinkering with).
Are there certain versions of M controllers where it does generate a code?