It would only take "hand tools" to assemble into a weapon, and also didn't have a serial number from the manufacturer. That's what ATF Agent Tommy Ho told me. The fact that I had stamped my own serial number and my name and city onto the plate did not matter to them. If I had bought a "registered" plate, (serial numbered by Halo) there would have been no problem, and they wouldn't have been visiting. Said that it was a semi auto plate, no problem there. Said that if it has been machined to the point that it can be assembled using only hand tools, as opposed to machining it, makes it a receiver. I asked him lots of questions, which he seemed to answer, but he didn't volunteer any information without my asking. I was polite, and so was he and his bodyguard. Fortunately, the plate was not assembled yet, so I'm only out the plate. I was using it to hold down 3 cases of 8mm so they don't roll around in the next earthquake. They do have Halo's records, and I'm in their home town, so that's why they visited me first.
Thats the facts, now the commentary.
My impression was that they want everyone buying through an FFL so they have a record of who has weapons. Home builders skirt this, and I'm sure that ATF feels unfulfilled by not kowing what everyone has. I can't recall the last time that anyone went berserk with a 1919a4 semi (or any registered machine gun either) so they wouldn't seem a high priority item to me. I told Agent Ho that just the week before, my son was at a gas station filling up, and in the garage bay was a car with the trunk open, selling handguns (no 10 day waiting period, the only background check being "How much cash you got, man".) He didn't seem interested at all. Didn't ask what town, what gas station, nothing at all. So now you know.