Chris - Looked though all my pictures - both Mustangs and early Shelby's just to see if I could find any engine from around that period with a label on that loom
1- I would expect that they did have a tag like just about every other loom on the car but can not prove it. Its a much larger loom than the later car engine feed ones.
2- Would expect it to be at the engine end of the loom rather than the firewall or starter relay ends but again - can prove it. Found this one shot but it looks as much a reflection as a label.
5R161xxx wire loom label maybe
Found a number of cars with labels still in place on the automatic trans wire loom and the forward engine compartment looms after all these years - not sure why the engine one didn't hold up as well
5R178xxx wire loom label at relay
5R188xxx eng wire loom tag (Charle's parts car)
Sorry - just what I found If it makes any difference the auto trans and the front looms both had white labels