[andrewcooke]

apart from all the other comments here, you seem to be confusing natural and surrogate keys. the "identity" of the quote (the quote number) is a natural key. the "identity" of the quotelines table is a surrogate key - the equivalent in the class model would be the address of an instance (the thing to which the pointer in the quote points).

there's nothing significant in the class model only having the one identity; it's just a natural consequence of what's implicit and explicit in the two technologies.