First IMHO don't try and determine what is correct by looking at other restored cars at a show.
Allot of times clips and retainers are removed, not replaced, added or simply forgotten about during ownership and restorations
You may have had a judge that was more comfortable or familiar with 65's so the clip looked "out of place"
Yes all plants used them during 1966 production
Short of doing a full survey of all plants all months here are some of the earliest examples I have documentation of (following the three example rule
NJ - 6T09T1023xx (Aug) - 6T07C1044xx (first week of Sept) -6T09C1144xx (third week of Sept)
SJ - 6R07C1032xx (Aug) - 6R09T1076xx (mid Sept) - 6R07C1097xx (mid Sept)
So its easy IMHO to see that both of these plants used the clips on 66 Mustangs built at their plants but on to the plant and year that should be our focus
As for
66 Dearborn I found very few unrestored examples with the clip. Not sure if this suggest that they were used on some cars/ time periods and not during others but looking forward we find that the clip was used during late 65 production at Dearborn and allot more examples found in those files
Findings include Dearborn - unrestored - 6F07C16970x and even back into 65 production at Dearborn with fairly well know car that Mike M owns 5F07C7806xx & 5F08A7275xx. Additional late 65 examples include - 5F07C7396xx - 5F07C7562xx - 5F07C8021xx -
So with what has been offered if you choose to use this research at the next show, during the introduction of the judges and if they don't if there is something different or important that you as the owner would like to offer before they start simply ask them - "Can I share something you found out during the restoration of your car?"
With this respectful opening offer that the question of the clip and the location has come up in the past, your research it, found the information above and thank them for considering the research then let them move on to judging.
Hope this helps
Don't know if this helps or not, but my car was built in San Jose with a build date of February 10th 1966, it's a HiPo, but it originally had the clip and it was green just as shown in the attached photo...
Jason - haven't found green clips on cars I've got pictures of or ones I've found in person. Only have seen the green on service replacements, and new ones sold today. Just offering an observation