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Martin Crossley

wrote:
For your perusal...

SNIP

===2 core wiring===
A small minority of houses still have 2 core [[lighting]] circuits.
Metal pattresses or accessories should not be fitted to such circuits
due to the absence of anything more than functional insulation as
protection against shock.

Some 1960s metal back-boxes had nylon inserts for the (4BA) screws.
The screws are thus insulated. They are fine as long as any
replacement switches are plastic and re-use the original screws.