The Model M4-1 numeric keypad was an optional coupling for the IBM Space Saver Keyboard (with or without TrackPoint II). It's essentially the same as the Model M3 numeric keypad except in a larger case and comes with an extra key. Without modification, the keypad can only be used whilst being coupled to a Model M4 or M4-1 keyboard via a modular 10P10C RJ-50-like jack.