Phil - You are correct in your undertone that none of us *really* knows, we are all justa trying to figure this out, and many of us are just trying to contribute clues and assemble facts, which was what my original write-up was for.
You are approaching the slide lockup question from a different direction. I just tried moving the slide slightly to the rear (simulating a case which was not fully seated) until the trigger would no longer give me that characteristic Glock click. Yes, the barrel drops at an incline, and does not make a big gap at the breech, but I think if you are starting ANYWHERE short of full design lcokup/closure, the slide will begin moving earlier, expose slightly more case, and the unsupported area on the case then may give way. kB!
This is just my theory after spending hours with inspection lights and trying to see just how the thing could fail like that. And (based on early reports that Glocks had no problem with hot Israeli, +P, and CorBon-type ammo), and considering they must contain a proof load, the pressure theory didn't seem to be a good fit. Incomplete lockup did. That was a puzzlement until I went back to examine the ammo I bought, and found that the "jackets" were suprisingly irregular. After some discussion on another board (Battlerifles or Amback, I think), it was suggested that this was not really jacketed ammo, just plated. Bingo!! That's what it looks like -- that, or the world's most wavy, irregular jacket. I know on some firearms, like M1As, Garands, and BM59/62 types, reduced headspace can cause interesting problems that look like something else. This happens when the tiny shoulder at the chamber neck cruds up. This sounds like a possible. Others thought the casing was not properly sized, but Glocks chamber is "military" oversize. I don't think it was the chamber, I think it was the slug, where tolerences are less precise with a cast slug getting an imprecise jacket, you have the option for tiny shavings to accumulate at the neck. This prevents complete lockup of the slide (probably a bad term, since it doesn't really lock like a rotating bolt would...), which exacerbates the unsupported chamber issue. If there was higher than normal pressure, this could also have been a contributing factor.... Again, these are only theories, but I suspect it was more than one thing that led to this, as these are thoroughly tested arms, and very few -- even of this earlier generation -- have had any problems.