After study, it appears that this was one a series of fills of a drainage channel in the area.
A probable layer of post-abandonment accumulation, which filled linear cut SU510 after it had gone out of use, possibly as a drainage channel. The presence of this layer of soil under stone packing SU535 and SU539 - and more specifically the mid-Republican pottery found in this layer - most likely precludes these stones from being wall foundations, and cut SU510 from being a foundation trench.