Sooo.... Seems like ATF has set themselves up for some serious confusion here. Just imagine if an eagle eyed ATF agent, expert in Artillery and Navy Lugers, says that your repro stock is so poorly made that it doesn't count as a repro.
The pistol/rifle dilemma seems to be a non-issue with the Lugers, but the AR-15s are just another example of ATF's confusing rules. The registration I'm talking about is not tied to the owner and I'm not sure how it really works, but I believe it's the manufacturer who designates lowers as pistols or rifles and then reports production numbers and serial numbers to ATF.
Now how in the world would you know if it's a pistol or a rifle when you pick up a stripped used lower at the gun show? And how would the cops know if they check out your AR-15? And most importantly: Why in the world would anybody ever need to know? Seems like the only thing ATF accomplished was to build a trap into the serial numbers: You buy a lower that happens to be a pistol and build a carbine on it, and you're breaking the law.