I have a 2018 +4 which came with three keys one of which I keep in a safe place.
As mentioned elsewhere the transponder in the key reacts with a wire loop around the lock. This wire loop connects into a black box up behind the dash that has been specially made for Morgan by Avonwood Developments Ltd:
[Linked Image]

This in turn connects to the ECU

So I think the answer is it is bespoke.
I have not asked my dealer but they may be able to program a new key

Tim
Plus 4 2018