System of book-keeping as old as the Ark? (6,5)

'the ark?' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'system of book-keeping as old' is the wordplay.
I cannot quite understand how this works, but
'of' could be 'o' and 'o' is present in the answer.
'book' could be 'nt' (New Testament) and 'nt' is found within the answer.
This may be the basis of the clue (or it may be nonsense).

'as' acts as a link.

Can you help me to learn more?