I wonder if the "hand built" factor is coming in here. Slight variations show up in a myriad of ways on a Morgan with varying positioning of parts etc. Could this be carried through to the windscreen assembly? If, as the history of this cracking seems to indicate, there are issues with screw length then a logical solution is to rigidly control the screw length either by measuring or introducing selectivity of screws in certain positions with some kind of failsafe check built in? A supply of short screws clearly marked/boxed for the corners perhaps? A different thread screw to make it impossible to insert an over-length screw? (Poke Yoke style) Attention to detail during design and building in controls to prevent mishaps at that stage saves problems later.
