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If I recall correctly, that piece of wood looked like part of a wooden shaft; it looks like something that would be used to probe up the shafts to see how far they go, like a sounding line used to check water depth. I don't think it HAS to have been in place since the building; IMHO, it was probably used by whoever first actually opened the bottom of the shafts, probing around to get an idea of how far the went into the structure.