The Customs website makes it clear that the 4457 form is NOT a requirement. Instead it is only one form of acceptable proof that you acquired the property in the USA and took it out of the Country. A dated bill of sale or sales receipt serves the same purpose.

Ted is correct though about the Minneapolis airport customs. I tried once to use my Canadian gun registration card to show prior ownership and the officer informed me that they would not accept a document issued by a FOREIGN GOVERNMENT! I told him to either let me go or go ahead and confiscate my gun as I had a plane to catch. He let me through with my gun...Geo