A couple months ago I ordered a percussion pistol lock, stock and 12" barrel, with some other fittings, from Muzzle Loader Builder's Supply in Idaho and could have just as easily got the parts from T-O-T-W. No hassle whatsoever - the package arrived at my post office and I paid the GST and took it home. Doesn't have to be registered until capable of being fired, according to the advice I got from CFC. A couple of weeks ago, when I went to register it on line CFC wanted to know the serial number on the "receiver", of which there was neither, of course. So I explained this and once they understood, they were happy to know that the barrel was from Green Mtn. and they will now send me the serial no. sticker to put on it or I can stamp the numbers into the barrel. What they seemed to want was to put the ID serial no. on the most prominent part of the gun and agreed with me that the barrel met that expectation. My long term plan is to get a second barrel and flint lock to interchange into the stock and for this I will get a second serial; no. for the flintlock barrel, since the ignition system is different. They seemed to want to know that the barrel was commercially made, rather than something I made myself (would that make me a manufacturer and subject to different rules, I wonder? Anyway - no sweat importing the pistol parts....
good luck.