Thanks, Knothead, for your reply!
The mast is down, and the spreader bracket on the "nut" side of the through bolt is off. But I can't remove the spreader bracket on the "bolt head" side of the through bolt without removing the bolt. And the hole in the mast is just big enough to pass the bolt through-it does not emcompass the outside diameter of the compression tube.
Though I can't move the through bolt in it's longitudinal direction, (?"athwartships"), I can jiggle it around a bit. (It doesn't seem to be fixed to the inside of the mast.) So I'm assuming there is a compression tube in there, the inside diameter of which matches the outside diameter of the through bolt, preventing me from sliding the bolt out, with a length of the compression tube equal to the inside diameter of the mast.
I'm thinking about checking out the hardware/auto parts store for some kind of "screw press", sort of like those used to remove stuck faucet handles, but more stout...