These stones appear to have originally been a threshold into room G-8 from G-1 or perhaps first layer of leveling on top of the opus reticulatum wall (6086) in order to build 6088 on top and plug the doorway.
Walls of Rooms G-1, G-7, G-8, and G-12
Photo from south
Photo from north
SU 6088, SU 6087 and SU 6086; photo taken from the north
Looking south