She's mint

Where does the phrase "She's mint" come from?

Coin collecting. Mint is the highest quality.

I hope the "she" here is a classic car rather than a person.

If she's a person, she's one grade up from Uncirculated....

Boys, boys, I think we're going in the wrong direction here!

Persons talking about old cars in the Western US might frequently be heard to say "She's mint." as a complement or a bostful expression.