Interesting study.
Since it's a little difficult to compare them picture to picture, it might be instructive to remove the upper from the frame and then trace them one after the other on a piece of paper, with a small, consistent horizontal or vertical separation, similar to the diagram Vlim posted.
It could be a bit time consuming, but might show interesting results. I'd also like to compare multiple examples of the same model and see if there's any natural variation in the position and curvature of the "cut" down to the main part of the frame.
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