As with the chimney in the northwestern bedchamber, the answer relates to the creation of the Drawing Room in ca. 1812.  To form the large room, a structural wall was demolished on the first floor and a truss partition was built on the second floor.  The mortises in the floor are actually the remains of the ca. 1764 stud partition that was located about an inch-and-a-half in front of the existing truss partition.  Because all of the mortises are the same distance apart, these mortises also reveal that a doorway was not found between the Northwestern Bedchamber and the Old Library in ca. 1764.