Small aside — In sparky’s last picture, porcelain knobs were usually furnished with a ~12-penny nail and with some—a washer of cut leather.
Nowadays, I imagine the high-brow construction junk shops—er, “vintage building restoration and antiquities” firms are probably selling salvaged knobs and tubes at 50 times their original price.
As far as “cutting-in” to existing K&T from modern nonmetallic cable, I have successfully {passed inspection by} using a 3¼-inch metal octagon box, with a clean ~6-inch length of romex jacket as “loom” for single conductors exiting the box. In these cases, just dogged off the bare ground with a standard 10-32 green/hex machine screw, for luckily there was no need to extend an equipment-ground conductor.
