The new additional photos are helpful, but w/o the original builders input, it would be just a guess on what his original idea for the mast base.
My best guess is the guy was fairly good w/wood and made a mast base w/an insert of wood
. He probably grabbed a second hand rig from a yard and fitted on this boat. Since it doesn't appear to have any fastener holes in the inner wood (hard to tell from the pics), the corrosion holes that you uncovered were probably from some old hardware
that was removed/not used on this boat.
From the approx. position of the wiring
insert in the mast, the wood insert goes up ~1'.
Does the inner mast base wood seem dry? What does the standing rigging
look like for this mast?