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After taking them apart I noticed that one of my two Atmoses has a 15 pin connector on the keyboard versus the usual 14 pin one - the motherboards of both machines have a 14 pin header so I'm curious on a couple of things:

1. It is safe to assume that the striped wire on the 15 pin connector should be pin 1 on the board header identical to the 14 pin connector?
2. Has anyone seen this before? Both machines are UK spec so I'm wondering what the purpose of the extra wire is?

I also have these 14 or 15 pin connectors.
We could give them a number from 1 (the nearest pin of the IC 4051, single chip on the keyboard) and go to 14 or 15...
The 15th pin is not connected. But be sure to always plug the motherboard's connector beginning with the 1st pin as described above.
If you have a 14 pin connector from the motherboard, so you should have the 15th pin of the keyboard out of this connector.
Otherwise, you could call Houston for some problems...
Double check and everything will be ok !

I'm curious what the reason for the 15th pin is - the Telestrat keyboard looks remarkably similar - perhaps the 15 pin board was a precursor to that to allow extended characters?

I've seen both of these connectors.
If you look at the Oric circuit diagram there is
PLS
0V
5V
0-7 columns
Rows AD 0-2

So that's 14. The connections are numbered 1- 14 with no 15 so I think it is a spare or one that was left free for a ground or screen, which isn't needed.

Most likely the reason is one of cost and/or availability, depending on supply conditions, only 15 pin headers may have been available or they may even have been cheaper than the 14 ones. In that case, Tangerine may have accommodated the design of the board to support both so they could order whichever was cheaper/available at the time.