Another socket puzzle: Why the extra terminal?

[Lucien Nunes]

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.

 

[John Arnold]

Come on Lucien get to it, your losing all respect here

 

[Deleted member 105166]

IP camera disguised as a non obtrusive switch to film certain shenanigans that are then streamed on the internet for a pay website at £19.99 pcm.

how did I know who wrote this post, just by the content, before checking the author was indeed @SWD ?

 

[Deleted member 26818]

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!

You don’t want to see my fingers, if you think yours are bad.