Peter Parry wrote:

By a masterpiece of technical superiority, between them the BSI and
BT managed to number the 431A plug in the British Standard as a
mirror image of the socket, so when inserted pin 1 on the plug goes
to pin 6 on the socket, pin 2 to pin 5 and so on.

So the answer to the question that has been a mystery to me for so long
(see my other post) is that the connector pin numbering came from a BSI
committee whilst the LJU terminal numbering was determined by BT, and
the two didn't talk to each other. (I.e. it was a cock-up.) Is that right?

The pin numbering isn't shown on the plug but many packets of plugs
in the DIY sheds come with a little diagram on the back which often
follows the BS numbering.

If you do the logical thing and put a plug on assuming the numbers
are as on the socket the phone may not ring. Just to confuse the
issue further some imported sockets also invert the numbering so if
you wire pin 1 on an NTE5 master socket to pin 1 on a Chinese (insert
shed name here) socket it may well be going to 6 :-).

A simple way of remembering the correct orientation for the BT plugs
if you like mnemonics is Ring on the Right. When you are holding the
plug with the latch on the right the ring terminal (4 on the plug, 3
on the socket) is always just right of centre.

All useful info, saved for reference, thanks. Of course none of this
would have happened if the numbering had been got right in the first
place. Plus ca change...

--
Andy