Take a sentence like:
'Excellent lady that she was, Lady Gigabytes invited us over for dinner.'
'Lady Gigabytes (excellent lady that she was) invited us over for dinner.'
What both these sentences mean is:
'Lady Gigabytes was an excellent lady so she invited us over for dinner.'
Why, then, don't we use an article before 'excellent lady' when we write 'excellent lady that she was'?
Thank you in advance!