There was a flat amored-type of cable available for burying in plaster. I have a Audels book that shows this, same thing "Wiring finished buildings". I've only seen it once, as recently as a 1940s house in a kitchen. I pulled on it and it snaked all the way up to the light fixture, where it tapped it's power.
To this day, the vintage hackjob that takes the cake for me is the bare steel wires run through an attic on spool insulators nailed to the rafters. The outlets that were fed off this circuitry went through K&T loom once they penetrated the walls. My words were literally "WTF?" At least the roof was steeply pitched and the wires were 7' up, out of accidental reach.
The story I heard from the old crusty next door neighbor was that these circuits were added during the war, when copper was needed for the effort. Original wiring was K&T from the 20's.
