Ross Herbert wrote:

The common GPO 3000 had the copper slug embedded into the armature.

No Ron, the 3000 type relay armature was fitted with a small brass
residual stud to ensure the armature was not unduly held by residual
magnetism in the core on releasing, NOT a copper slug.


You are probably correct copper/brass non ferrous anyway


impulsing relays used an adjustable residual stud (brass screw and locking nut)
so that the residual gap could be set according to specific
requirements.

I imagined that was to adjust the throw of the armature to provide and
also take care of the residual hang.


Here is a pic of a 3000 type relay with a 1" heel end slug and an
adjustable residual stud.


I don't recall seeing one just like that, all the GPO types we used had
the coil full length of the frame, some did have a D shaped shading pole
set into the armature end of the pole.

We also used a later type of GPO relay, exactly the same in design but
slimmer frame and coil.

Life was so much simpler back then: GPO relays, Bulgin microswitches,
Crouzet motors, Honeywell timers... Selenium rectifie... erm well praps
not!


http://www.englishclocksystems.co.uk/slugrelay.html
Amazingly, I have one of those slider rheostats in my workshop right
now. I converted it into a dummy load for testing power amplifiers.

Ron(UK)