Yes, they replaced the inner liner material that they had used for 15 years. The inner liner material used to be Barex, but they moved away from it because it was becoming obsolete.
For a while with the pressurized well, introduced in the spring of 2016, they used Mylar sheets with no inner well. Just pressure to hold the book between two sheets of Mylar.
They replaced the two sheets of Mylar with an inner well again but I can't remember what it was made of (whether it was still Mylar or something else).
The Newton Ring effect was present on both holders with the latest design - the one using Mylar sheets in the spring of 2016 and the replacement about 3 months later (around June 2016) that we currently still use.
That's what leads me to believe that the new inner holder might still be Mylar.
Basically correct. It was Barex-brand polyacrylonitrile (PAN), and this material was becoming obsolete to other materials. Eventually, one of the last companies manufacturing it (Ineos Group) stopped and closed its plant in Lima, Oh because they could no longer turn a profit with such low demand. When this happened, CGC lost its supplier.