Your R4 looks just like the photo in #10 of the linked thread of post #2. It appears the steel outer shell slid forward, which let the O-ring extrude. The only thing that holds that shell in place is a little tab that folds down against the aluminum inner body. You could conceivably fix it in-place by just reinstalling the O-ring (or a new one) in the groove, sliding the shell back, and securing it. Perhaps the lock tab broke off and the rebuilder's didn't weld on a new one.

There is a place for a pressure-relief valve, which might have been installed in some GM apps, but in ours there is just a plug w/ snap-ring there. At least, that is what I recall from past posts here.

I doubt anyone would have been hurt had they been working in the engine bay, unless it scared them to bump their head. Worst would have been a face-full of refrigerant oil.

The high-pressure port is well hidden on the tube that runs up behind the alternator. But, the factory procedure is to fill a certain weight of refrigerant (3 cans, I recall), so unlikely they would have over-filled it. Seems there might be some less than ready-for-prime-time rebuilt R4's in the supply chain, perhaps traceable to some Billy-Bob assembler who doesn't understand them.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans