Thanks for that, Jonathan.
Here's an idea, building on that classy cone idea JonJo linked to, for securing a shackle with a countersunk-headed pin so it will NOT unscrew, until (if you're lucky) you want it to:
Make the cones with a sufficient 'land' at the small end (axial face) to butt up against each other and resist the shackle ears being squeezed together
Make their length so they only just assemble into the shackle.
Use high strength retaining compound Loctite, not just on the threads but in the countersunk hole and the mating face of the pin (countersunk screws, snugged up against an immovable face, are NOTORIOUSLY unwilling to unscrew again, even without Loctite)
To disassemble, first heat the Loctited parts
with a hot air gun (to at least 165C, 330F)
Holding the body in a smooth-jawed vice (or a vice with protectors) try loosening the pin. It will probably still be immovable.
Now for the old toolmaker's trick, handed down for generations:
Take a short pin punch, the largest size which will fit in the hex recess for the allen key. Support the underside of the ears (on the same side of the shackle as the head
of the pin) on something heavy and metal (storm anchor
, vice jaws) and use your biggest hammer to firmly thump the pin punch end-on, smacking the countersink against the mating face.
A few decent thumps should do the trick. If not, get the grinder out with one of those 1mm thick cutting disks, and cut through the shackle legs in two places. You can always weld them back together later (just kidding)