Because it appears to have been arranged in its position before the placement of drain 7246, this feature appears to be related to the construction/use of the drain 7201/7196/7246, perhaps delimiting the vertical shaft in which the drain emptied. Its function and relationship to the drain will hopefully be clarified by further excavation (necessarily entailing removal of at least part of this feature), which might reveal the construction cut for the drain.
Sketch of SUs 7255, 7256, 7258, 7259, 7260.
photo from north
photo from west