Ah, that's where they are to be found! Now I feel silly...
It's surprising how blinkered we become, even using PSu most days, but in some select areas only.
I never use Catalog Image Details. As custom fields are initially created in the Info panel, that is where I manage them.
Now all problems are solved as I can mark each custom field as a favourite and then easily drag and drop them to a newly created private label.
Thanks a lot! I can now stop pointlessly attempting to create scripts to find them.