I betcha that it is contact rather than mylar. What I've found is shrinkage like that (around the edges) is a dead give away. Mylar is thicker and does not shrink THAT much - just my thoughts.

Many machines of this vintage have an easy mylar/contact removal. I'm surprised you have not tried the hair dryer method ?

Looking at that playfield - there are some sections that would have been tricky - but you have the skills to restore it - so keep the updates coming !