(1) When a paper whose contents at the time of signing show that it is intended to become an instrument is signed while still incomplete in any necessary respect it cannot be enforced until completed, but when it is completed in accordance with authority given it is effective as completed.
(2) If the completion is unauthorized the rules as to material alteration apply (Section 3--407), even though the paper was not delivered by the maker or drawer; but the burden of establishing that any completion is unauthorized is on the party so asserting.