Thanks for sharing,
What a great story and how exciting for everyone involved!
That you've researched and flown out to visit these prior owners is a testament to your, and their dedication.!

The day Carl sees the A12 will be special no matter what you do..
- You'll take him for a ride.. I assume you'll let him drive it himself too? ..assuming there's nothing preventing him from doing so..
- Birthday presents - There must be some decent 1:18 scale die cast A12's.. can you get one in the same colors, or have it painted to match, give it to him?
- There will be plenty of photos taken at the time, but how about printing and framing some really nice photos beforehand too and having them framed for him to take home.
- Buy some vintage A12 brochures? Are there any memorabilia from the dealership?
- You could name your event something and print up some T-shirts.. mimic some vintage promotional material on the T-shirts? (Beware there can be a lead-time to print T-shirts).
- The weather looks nice where you are - so book a group lunch at an outdoor restaurant you can drive to, invite some other Mopar people..
- Invite the local newspaper.. make a story out of it.
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Cheers!
- Art

PS - I was able to track down the original owners of our '65 Satellite, but never connected.
I was able to reach their pastor (there was a Sunday service program in the glove box). He was kind enough to accept and forward a letter to them on my behalf.
I included photos, and some xeroxes of the purchase documents from their time that stayed with the car.
Never heard back, but I'd check around online every now and then.
Over time, both the husband and wife's obits appeared.
I have their sons' names on various gas station and service receipts, so I know they drove it some, but never took it any further.