Each of the Target buildings came with a certificate of authenticity. I'm not sure how much the certificate counted for anything, since the items were not numbered, nor were the items identified on the certificate.
Each building also came with a registration form. If you filled one of those out and marked the buildings you bought that year, they sent you a limited-edition, numbered piece. I got four of these special pieces, but I don't know how many years they offered them, so I don't know if I got all of them. I will post photos of those pieces at a later date.