Problem solved, but not without more frustration. Drove down to the specialty tool store, and they had 3 of the wrenches in stock. Bought all 3 for less than five dollars. Good brand - Bondhus. If they worked I would save one, give one to the client, and give the last one to a friend in the business who might need it.
The wrench fit loosely, not quite fitting the flats properly. So I took one of my 4mm wrenches and started polishing down 3 adjacent flats to get it down to size. 3.9mm works perfectly. Loose production standards strike again.
Letting it sit overnight under tension with new strings, and will complete the setup tomorrow. This wasn't the adventure I wanted, but that was never promised me. Perseverance pays off.