Difference in stonework between a composite lock (L) and normal lock (R). Composite lock is Lock 71, regular lock is lock 33. This is a montage of the photos (PD): Lock_71_on_Chesapeake_and_Ohio_Canal_from_HABS.png and Masons_marks_on_C_and_O_Canal_Lock_masonry_from_HABS.tif