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I contacted the builder and also the mast designer about this construction - and here's the real scoop on it
From the mast designer:
The keel was likely designed that way because these boats were built over male forms. The keel stub--the wood part--was likely NOT part of those forms. Yet, the keel serves two functions--it has to hold the ballast down low for appropriate stability, and it has to have enough area to balance the loads from the rig. If you had a keel that was just the lower ballast part without the wood spacer, it would not have enough profile area, and the ballast would be too high up--center of gravity too high. Or, if you made the wood part also out of lead, there would be too much ballast. Or, if you reduced the thickness of the solid lead keel blade, it may be too thin, and the ballast weight would not be low enough. In a normal female molding process, that wood stub would be made out of fiberglass and the inside would be hollow and a deep sump. In this case, because there was no female mold and the stub was not included in the fiberglass hull molding, the next logical form of construction is a wood spacer with the ballast part attached below that.
From the builder:
Yes the wood spacer is to lower the CG of the keel. The structural integrity of the hull is simpler and better without a sump or the lead being poured down into a glass shaped keel. You can hit something down low in the lead or up high in the wood and your will have an easily repairable ding.