Maybe the butt of the mast was corroded and a previous owner cut off the corroded end and used that wood piece to raise the mast step up to compensate and allow full adjustment of the standing rigging (due to shorter mast, rigging would have been too long). OR, maybe the rigging was replaced at some time and was incorrectly measured, resulting in the cables being too long? Either way, that wood piece should have been protected with paint or better yet epoxy to prevent rot.