I took a look at both my San Jose built Fastbacks (a 65 GT and a 66 GT). The 65 (11S scheduled, 11 May 65) has two clips, one at each end, and the remnant of a piece of tape about 1 inch wide and 1 1/2 inches long stuck in the center (as in the picture above) that does not seem to have a function. The 66 (18K scheduled, 18 Oct 65) has three clips, ends and center (not as specified in the drawing), no tape.
The 65 Mustang Electrical Assembly Manual (AM0008 pg 13) specifies 372303-S100, 6 each, 2 across in the trunk, and 2 each per side for the cable to the rear quarter lamps. The 66 Mustang Electrical Assembly Manual (AM0013 pg21) specifies 372302-S100 (a different part number), 5 each, 2 across in the trunk, 2 for the RH wire to lamp, and 1 for the LH wire to lamp.
372303-S100 is slightly larger (ref AMK 2014 catalog, AMK B-10723) than 372303-S100 (ref Ford Standard and Utility Parts Catalog - 1969, Osborn AM0205, pg39).
Jim