wrench1: Thanks for the link to all those NTSB documents. They make a lot of information available to the public, which is impressive from an openness perspective. The discussion about the torque measurements on the remaining nuts/bolts following the accident is indeed in the "Materials Laboratory Factual Report 22-071" along with further detailed examination. About the torque of lower right nut/bolt being measured at 240 inch-lbs vs 360 inch-lbs required minimum, be patient with me: Isn't it possible the bolt would have plastically deformed (stretched) slightly in the process of the tail boom pulling the entire longeron attachment fitting from the longeron? If the bolt stretched in that way, then the remaining torque when tested would be less than what it had been prior to the failure simply because the bolt isn't pulling the nut up against the washers as much as it did beforehand. However in any case, NTSB doesn't seem to raise their torque readings as a direct issue*.
I commented on your comment "after the upper
bolt failed in tension the remaining bolt shank captured the ring as the boom fell away" as I think you are proposing here that the broken part of the upper left bolt was still in place and tore away the aft fuselage bulkhead (or fuselage tailboom mount flange, or ring). But isn't it a case of the other three attachment bolts, still being intact, tearing off their respective fuselage longeron attachment fittings and in the process, these then peeled the fuselage bulkhead off the fuselage?
* I had an indirect involvement in bolt pre-tensioning calculations for a non-aviation application years back. From memory, both too much pre-tension and too little were not good from a bolt fatigue life perspective if the application was subject to cyclic loading. So there is a 'sweet spot' that the fasteners should be torqued to.