Uhh Ted, what possible use would a Form 4457 be to Berrien Moore in this situation? He bought the the Darne in Paris. He's never imported it to the USA. He could never have taken it to a customs office to have a Form 4457 signed.

Your quote as set out above says to me that the customs form is the only method of proving prior possession in the USA. That's simply wrong. A receipt or bill of sale is sufficient proof as set out in the Customs FAQs I quoted. The Form 4457 is only one convenient method of such proof, not the only proof available...Geo