SU 5150 (block) appears to be sitting on (and in, on the east side) a 'dump' layer and may have been associated with a similar collapse of a northern wall like 5147 and 5148 and possibly 5149 or is a component of a different monumental feature. May be associated with the single grey tufo block immediately to the west (5151), with which it is roughly in line.
Single block east of 5149