The Special/Skylark/GS cars were built in 4 plants. Flint, Baltimore, Kansas City, Fremont.
Flint tags will have FB next to the body style
Baltimore tags will have BAL next to the body style
Kansas City tags will have KAN next to the body style
Fremont tags are unique, first they are the only tags that state "Flint, Michigan" on them.
They have an older style with rounded corners, the other three tags had a new, updated style.
They state "Magic Mirror Acrylic Lacquer" at the bottom
All the ones I've seen have a hand stamped number in the upper right corner
There will some kind of hole punched near the hand stamped number, I've seen 3 or 4 different shapes.
I'm sure I'm missing something, but thats the basics of how you can tell the plant from the tag.
Mark