Dear Dan, Jim, and mid-60s Buick "perfectionists,"
I understand your quest, but I have to confess . . . . I have the feeling you are
"goin' about it da' hard way!" When we bought our trusty wagon back in 1968, she had license plate frames from Hayward Dodge, which is I suppose where she was traded in (
oh da' shame, a respectable Buick traded in for a measly cheap old Dodge!

) She had those frames for decides. We never thought to replace them! I'm thinking about having some custom frame made that have the make, model, and year of the car!
Sure it would be killer to get original frames from where the car was sold. But until you find that, it might be better to put some frames that clue in an everyday person on the street that they have just seen a 50+ year old classic!
Just a thought . . . .
Cheers, Edouard