One of the forum's regulars kindly sent me a Data-General branded external numeric keypad unit, Model 2568. Looks to be unused (no dirt, not even on the feet). It connects via a stereo mini headphone plug (2 circuit). I can't figure out for certain if this is a DG One add-on, but the key styling is correct. I note that on some DG Ones, there is a little trap door on the bottom of the unit. I'm wondering if this is where the thing should plug in.

At any rate, I'd at least like my guess confirmed!