I have come across the lead sheathed wiring before, the rubber insulation inside the sheath was in very good condition considering it was nearly 60 years old.

Also have found the IRV cable in oak or other hardwood capping as described by Trumpy and Paul, had it on the lighting circuits in our old house until 1990, when it shorted out above a light fixture and nearly set fire to the attic [Linked Image]

I left some of the capping in place, the grooves were just the right size for 1.5mm Twin and Earth [Linked Image]