I still maintain that the 4457 is only one way of proving prior ownership; the customs website specifically says this. I usually just bring a copy of my bill of sale/receipt in case US Customs gets frisky.

Where I live it is not easy to visit a customs office to obtain a signature on an 4457 form. I cannot confirm the legality, but I've had no trouble getting my local postmaster to sign and postmark the 4457 as an ex-officio customs agent. No customs agent has turned it down so far...Geo