The 6 has been missing a nut for who knows how long. The nut in question was for one of the four carriage bolts holding the wheel to the headstock, the lower one on the outer side of the ring. Its absence was discovered by Marguerite as she inspected all the bell nuts during the Boat Race Saturday tower cleanup.
The nut in question is 12mm1.75, fitted by a 19mm wrench, like the stay bolts. A replacement was obtained (along with a spare to have on hand) at the local hardware store and installed before service ringing this morning.