This is an old British Postal key.
The G.P.O. made keys themselves in a works
not far from the Silvertown Telegraph Works in London.
They did not mark their keys. This is a very big and heavy piece,
measuring at the base 8-1/4" by 4-3/4" and its weight is about 3-1/2 lb.
The round brass bezel with its glass lifts off for adjustments.
This is a "double current" arranged key with two sets of contacts.
"KEY diagram"
Here you see a close up diagram of the key itself.
These keys were first made around 1880.
Brass finished keys were used at home,
while nickel finished keys were used in the Colonies.
The key required for this is quite specialized.
It is literally two paralleled keys,
the polarity of both must be switched between transmit and receive.
While it makes for a more complex key, it reduces the number
of relays by half since the distance a signal can travel is doubled.
"System diagram"
The British landline service used a different approach
than the American to extend the distance over which a
landline signal could be sent. They used a double current system,
doubling the voltage difference between the make and break.
Some detail of this system can be seen in the diagram.
