Yes. Also there is enough play that the 16 mm wrench flats mostly bear on the opposite corners of only one of the raised areas. I think you have given me some good ideas to try and I appreciate your help very much. I was thinking the socket I mentioned could be made to attach to the axle but your ideas are simpler and don't require a special tool.

BTW, I'm repurposing the bike to encourage a 15 year old boy to start broadening his fitness interests. I hope it takes root and perhaps will help him to put some of his baggage in the right compartment, so to speak.