Plaster lining the north side of wall 6401. Seemed originally out of place because it seemed like the corridor behind 6401 was outdoor space. Because wall 6105 continues north and there does not appear to be access from the NS road to the east, our current interpretation is that this corridor is a large drain, running into a known drainage feature in area F. Therefore, the plaster we see is likely an opus signinum lining for some kind of water management system.