David Hansen presented the following explanation :
On Sun, 17 Dec 2006 13:54:53 GMT someone who may be Harry Bloomfield
wrote this:-

Apart from the fact that it would be unlikely to have been wired with
an earth, those which were provided with one were just bare copper
wire.

Or alternatively the earth connection was made via the screws from
the conduit.

.... and the (likely) grip conduit joints will be so rusted that it
would no longer be safe to rely upon it for earthing purposes.

--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk