How much mass does the adaptor have? Is it possible that
someone started with it several turns loose on the spindle and turned on
the motor? That could rather firmly wedge it in place.

I have no idea. The mass isn't that great, but I imagine turning on
the motor would indeed wedge it pretty tightly. It's also possible
that the previous owner Loc-tited the adapter on.

My problem at the moment is getting something that will grab the 2"
dia. flange on each end of the spindle without slipping. (the adapter
also has that 2" dia flange) I tried cutting a 2" hole in a 2x4 and
clamping one of those on each end like two wrenches, but the 2x4's
slip. I'm temporarily "gluing" the 2x4's on with rosin and alcohol
and hope that that grabs enough that I can put some torque on the
adapter.

RWL