:
: The backstay was attached to the mast by a $150 high strength shackle
: (picture below). This beast has a 28K lbs working load and a breaking load
: of 75K lbs Apparently the rigger screwed in the pin but didn't wire it
: closed. Some time during the trip the pin worked loose and the shackle
: lost it's holding power and bent open.

Must not have been a Wichard self locking unit. Is that a Harken block beside the shackle? Was the shackle Harken as well? I work as a rigger and these details are important to me professionaly.

Just another reason to love a lashing!