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Relax and enjoy your new pistol. The ATF is not coming to inspect in what configuration your lower left the factory. If you legally bought the weapon as a pistol you have absolutely no criminal intent or negligence. The ATF is not the Big Bad Wolf that many law abiding gun owners make them out to be.
But if you are concerned about the issue the suggestion above of buying a new stripped lower and switching the parts just to be safe would be a good way to ease your worries.
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The ATF never comes out on it's own. There are however thousands of different scenarios that they would be called out.
How about your neighbor call the police because he sees someone breaking into your house. Cops come, finds the pistol either with the suspect or in your house, calls to check just because thats what they do.
Girlfriend is pissed at you and calls the cops saying you hit her, cops come out, see above.
Many more possibles. Too many. Good news is if you did not know it was a firearm that needed to be registered , as in the OPs case (keep the receipt), you will not be prosecuted(
Staples v. U.S.). But the firearm will be confiscated.