This block, though positioned at an unusual angle to the surrounding walls seems to be in its original construction position. Maybe it is the foundation lowest course for a decorative feature in the corner of room 19
Tufo slab floor drain cover west of block 5586 (5739), Tufo step threshold west of wall 5574 (5740), Continuation of east-west ashlar wall west of 5740 (5741) and Single ashlar block oriented diagonally north of wall 5741 (5771)