This post is interesting because it brings to light rumors and beliefs about what is "correct" for our cars. As I strived to be concours correct on my GT/CS, I studied various details to insure my car was as close to original as possible. I found the 1968 Mustang Assembly manuals to be extremely accurate and the Electrical manual clearly shows no grommet for the hood. The reference for this information is found in the Electrical Assembly Manual entitled "Turn Signal Indicator Models-All", page E8-8135-1, date 1-22-68.
Also, the photographed grommet advertised on ebay is clearly advertised as a trunk grommet, not one for the hood. I would never question anyone's advice, as everyone on this site is trying to be helpful and provide the best information possible. My only point is to utilize any and all documentation to back up any questionable issue.
As for the grommet in question, you can buy these grommets from NPD (#14603-1K) for $1.00. They are made for the wiring harness on the inner fender of a 1965-66 Mustang. My 1966 Shelby has this grommet and it works great to stop any chaffing of the wiring harness.
Just my two cents,
Russ