My boat is the 6th or 7th fiberglass GB42 built and may have a different system. From what I see, the corners are mitered and not ship lapped as you describe. I can see a crack in the paint to indicate a 45 degree joint. Were your joints at an angle at the corners?
Our GB36 is from the first batch of fiberglass boats built, so I suspect our window frames are identcal to yours. Yes, there is a 45-degree joint line visible in each corner of the frame when they are stripped. I don't know the name of the joint, but it you take the frame apart, one piece is cut half the thickness at the joint and the connecting piece is cut to half the thickness at the joint (but the other half) and the two halves are mated together. There is a a small screw right next to the 45-degree joint line with a plug over it. The little screw under it simply provides additional strength to the joint, which is glued together.
We discovered this when we removed the frame of the first window we ever overhauled. (We have since overhauled or partially overhauled 20 of the 23 windows in our boat.) I removed the four corner screws and plugs only to find they had nothing to do with holding the frame to the house. So on subsequent window overhauls we have left the corner screws and plugs in place.
The only time we have ever had to dissassemble a frame was when we found rot in the lower frame piece of the big opening window over the galley. We made a new frame piece using the same kind of mahogany American Marine used and reassembled the frame.