I cut the notch in the end of the stainless steel bolt at the bottom of the mast.

With the cold chisel inserted in the notch and the pipe wrench's jaws on the shaft of the chisel, I turned the bolt. It turned easily.
Too easily.
I looked inside the bottom of the mast, where the bolt passed through an aluminum tube.

The tube was turning, too. So I got out the reciprocating, Sawzall-type saw and, after a half hour of work and two saw blades, cut the bolt through.


If you look carefully at the ends of the sliced bolt and tube, you will be hard pressed to find where the stainless steel ends and the aluminum begins, so closely have the two metals shared molecules.
Now that this riddle is solved, it's on to stripping all the old paint from the mast. I was going to repaint it, but that would require removing all the hardware which, I assume, is just as welded to the mast as was this bolt to the tube. I'm looking into the virtues of leaving the mast a raw aluminum. I think I've found a substance that will stabilize the surface. If so, this chore will soon be completed.