(1)    “Incomplete instrument” means a signed writing, whether or not issued by the signer, the contents of which show at the time of signing that it is incomplete but that the signer intended it to be completed by the addition of words or numbers.

   (2)   Subject to sub. (3), if an incomplete instrument is an instrument under s. 403.104, it may be enforced according to its terms if it is not completed, or according to its terms as augmented by completion. If an incomplete instrument is not an instrument under s. 403.104, but, after completion, the requirements of s. 403.104 are met, the instrument may be enforced according to its terms as augmented by completion.

   (3)   If words or numbers are added to an incomplete instrument without authority of the signer, there is an alteration of the incomplete instrument under s. 403.407.

   (4)   The burden of establishing that words or numbers were added to an incomplete instrument without authority of the signer is on the person asserting the lack of authority.