Pertaining to ONLY the plastic harness holder that SOME examples have on the cowl, I haven't noticed yet that the later 67-68 version with the plastic sheathing has ever used this wiring holder. Some examples do not even have a wiring holder on the hood itself either. Not sure what this means at this time but it looks more like some workers may have simply tore off the holders and didn't use them or some suppliers didn't put them on the harness. The holder when present, looks to be secured with the ground wire. This could be the "why", maybe there were issues arising with poor grounds for the lights?
Here are copies of pictures a few people have shared so far (this thread & other thread):