That arbor nut would seem to be special. I cannot find a socket to
fit on it. It looks like a 7/8" would fit, but it's just a bit
small. 7/8 is about the largest one can find for 3/8" drive. If I
go to 1/2" drive, then I run into the case where the walls of the
socket are too thick and do not fit inside that spanner nut.