I've heard a lot of people say these have a zipper top to them. Which is usually a result of a newer mold. I am certainly not in the know about this, but it seems unlikely that a change is plastic would make them have that property again when no other Firebirds do anymore.
Could be the new plastic, but it seems like there could be a new mold piece.
Just got 25 in our custom order. All embossed. None had a zipper top.