Propane cylinders can operate at up to 320 PSI, and their burst pressure is much higher than that, so its not unreasonable that an idiot could put a significant pressure of oxygen into one.
Also acetylene forms explosive compounds with copper and several other transition metals that are extremely sensitive detonators (to the point that they are too sensitive for practical use) and has the widest range of explosive limits of any commonly used gas
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You absolutely do not re-purpose other valves, fittings, regulators, hoses or piping for acetylene due to the risk of explosive metal acetylide formation. That's probably what finally triggered the timebomb the deceased had constructed. He probably got away with it initially because there was no metal acetylide buildup. Leave it a while then move any part of the improvised valvegear and a detonation was the most likely outcome.
Acetylene cylinders need that porous packing to break up the volume of the headspace enough so that if it starts decomposing in one pore, there isn't enough energy released to overcome the small activation energy barrier and start decomposition in an adjacent pore. Without the packing or some other means to rob energy from the reaction quickly enough (e.g. cold walls of a narrow bore pipe) decomposition propagates explosively. The *ONLY* place its permissible to have acetylene gas at a pressure above 15 PSIG is in the pores of the packing in the headspace of the cylinder, and in the short narrow passages of the cylinder valve and acetylene regulator. *ANY* impact that causes the cylinder to 'ring' enough to compress the packing is a really bad idea - if the culmunitive damage compresses the packing enough to open up a significant void, you've then got a timebomb sitting there
As for the level of stupidity involved in mixing *any* fuel gas with oxygen under pressure in a cylinder - well I don't understand how the deceased made it to the age of 39 without getting killed off by some other hazard. To even contemplate mixing acetylene and any oxidiser other than at the tip of a purpose designed and properly maintained acetylene torch or burner is just plumb crazy.