Problem solved! Boy, am I dim. Here's two pictures: the RH switchgear complete (for ID) and the kill switch removed. There's a small retaining tag at each corner of the kill switch body, two of which hopefully can be seen in the attached photo. The red slider knob and the thin black platform below it are both external, while the tags are inside the twistgrip housing. Working from inside, push two tags on one long side inwards with a small screwdriver, keeping upwards pressure on the switch so they clear the housing. Repeat on the other side to free the switch, then use the screwdriver tip to put local pressure on each of the body's copper rivets so they too clear the housing.
Thanks to you all for taking an interest, and I hope my dimness can help someone someday!
Standingtree