There is but one king, and I am one.
Here, the 'one' at the end of the sentence stands for 'one king' and 'one' is not specifying (in this particular place) a numeral. It is implying 'I am that one' or, more specifically 'I am that one king'.
Therefore, should 'one', when used in this way, not be considered an article with a well deserved place in the much documented sequence :
zero ... some ... a/an ... the ... null
And, if so, where in the sequence should it be inserted ?
Zero example : Who would wish to be king ?
Some example : There are some kings, it is so.
Indefinite example : I am a king and everyone knows it.
Definite example : They respect the king, thus they respect me.
Null example : Many wish to be king but the fact is that I am King.
Reference 1 : Stack Exchange - Why is 'any' not an article ?
Reference 2 : Acquisition of the Zero and Null Articles (Peter Master)
I think "There is but one king, and I am one" is a mistake, at best. There are complicated circumstances in which that could be forced to work and was that what you were asking?
"There is but one king, and I am he/him/it“ work because they don't misuse "one."
Quite clearly without a “that”, the final “one” does not stand for “one king”, nor is it implying “I am that one (king)”. Your search engine of choice will demonstrate that by failing to find examples.
– Robbie Goodwin May 23 '18 at 22:57