Hi: P. Raz... It's my understanding that Trillium
...the "Original" that is, used plywood on the inside of the fiberglass. This became a big problem if window leaks
weren't looked after in a timely manner. Wood doesn't bode well when wet!!!
didn't use anything but pop rivets and butyl putty and trying to put a flat window in a curved wall has its own set of problems!!!
Here's what I found after taking the small vent window out of the kitchen wall of our '77 Boler
Alf S. North shore of Lake Erie