Further investigation has revealed that there is a slot in the casing for a magnet in the rear diff housing so I suspect that the magnet fell out of the slot while I've been moving around the dismantled bits of the gearbox, and it stuck to the nearest steel thing which was the torsen diff. The three from the ATF sump are still missing.
Certainly whoever 'rebuilt' the murple gearbox never bothered opening the rear casing or replacing the bearings as they are very loose.
The torsen housing coming off the PF box:
Nice clean torsen. You can see the slot for the magnet directly at the bottom of the casing. The magnet is still in place in this one.
PF end housing on the left, murple housing on the right. You can see the two bosses on the right hand one which have been broken off the left hand one.
320k mile never-opened casing with bearings and spur gears removed.
100k mile probably-opened casing with the bearings still in place. These will all be swapped in to the murple's casing and reassembled.
