This is the second time I've encountered this flaw in our old Tril and so suspect that it may be common to many. There have been numerous posts about leaking Trillium windows
and how to rebuild the frame and reseal them, so I won't repeat that information here. But two years ago when I did the rear window, I discovered that the original window screws on the left side had missed the fibreglass all together. It appears to have been cut too big! The screw holes on the other three sides were a good quarter of an inch onto the glass, pinching it well to the wood. And guess what? The side with no screw holes was the side that had always leaked. This year I did the front and it was the same story. In both cases I improved the seal by first adding about 5/8 of an inch of new fibreglass to the bad side, making the opening smaller. That made it a little tricky getting the window back in but the screws now pinch fibreglass to plywood all 'round as they should. This doesn't take a lot of skill as, if you are careful, the lip of the window will cover your work. Just be sure the finished surface is straight with the body and "pretty smooth" so the butyl can do its job. Oh, and the good news: neither window now leaks!