Another socket puzzle: Why the extra terminal?

Here's an old accessory made to BS1363. You can connect it to a ring and plug 13A plugs into it. What is the fourth terminal for?

I've blanked off some text moulded into the bakelite so as not to give too many hints at first. Nothing else is hidden.

 

You need to stop biting your fingernails

 

Some form of switch?
The socket is energised only when there’s power to that terminal, lower current than the ring?

Just a guess

 

Is that a fused output?

 

Yes. What for?

 

You might be onto something there, as the cable hole size is smaller than the others..

 

Fused output for e.g. a round pin lighting socket?

 

I was going to go for clean earth?

 

Ah... 13a socket and a 5 amp on the same plate... for all the old lamps that people had plugged in

 

Surely the point of BS1363 was that all appliances had the same plug, eliminating other kinds of socket? A lighting point controlled by a wall switch, fed by the lighting circuit, might reasonably use a round pin socket to avoid other appliances being connected. But this is off the ring, so why not use more 13A sockets?

 

What's a a clean earth?

 

One with no polluting human beings on it

 

deleted, need more beer.

 

Show us the front then.

 

I ripped chunks off both thumbnails ages ago using them as emergency screwdrivers, but my hands have taken such a beating with working round the clock for months that all my finger tips are basically worn through the skin and the nails worn and split. I was even contemplating wearing gloves which I hate!