A guy I worked for owned a Seattle Computer Gazelle that he said "might be the one that DOS was written on".

(Maybe he meant the model, rather than the exact serial number? But he was in the PNW, and I understand had been very involved in the early microcomputer scene there, so I wouldn't be surprised.)

He was actively using it when he said this, so it still worked at the time.

If he still has it, some company that built an empire atop DOS should find some money in the sofa cushions, and make him an offer.