How about the equipment exchange board on this website?[:D]
Be aware of this though. It doesn't matter one bit when the reciever was made. What matters is that it was a complete rifle as of the cutoff date. So unless the seller can provide hard evidence that the thing was a complete rifle as of the cutoff, it's not a preban.
There are a couple of types, like the Colt SP1, which were never sold as anything but complete rifles, so if you get one of those pieces you're safe by definition. But if you get a lower made by any of the aftermarket folks, who sold stripped lowers, like Sendra; you're going to nedd the proof when you buy it.