It's a bit easier than that...
Maybe a better way to look at it. (Although I'm flipping through all my courtesy BATF FFL manuals to make certain I'm not running afoul of the latest and best regulations)
Aforementioned 1911 receiver/frame began life as a pistol frame (Pretty obvious since there aren't many factory rifles sold on 1911 frames).
If you slap on one of those carbine kits that replaces the slide, you've created a long arm, right?
True, to a point. You've made a long arm out of the 1911 receiver, which is totally legal, assuming that you stay within barrel length limitations, total weapon length limitations, 1994 Crime Bill Assault Weapons criteria, etc. But that receiver/frame is still marked, from the factory, as a pistol receiver/frame. So you're entirely in your legal right to return it to a pistol configuration at any time, no problem, and no penalties under law. Nice option, ain't it? You can go back and forth from pistol to long arm all day long, much like the Thompson/Center Contender and Encore family of guns.
How's the average Joe supposed to know his particular receiver/frame was marked at the factory as a pistol or rifle? Not too easy. Olympic Arms had a catastrophic fire at their facility that destroyed their manufacturing and serial number records up to the date of that fire. Some may interpret that fire as a good thing when it comes to identifying pre-ban AR bare receivers from pre-ban complete AR rifles built by that company. Likewise, when it comes to identifying which AR lowers are marked for pistol or rifle use during that gap in coverage, who knows?
How about the guy who gets one of those 80% finished AR lowers, and finishes the milling/drilling/machining on it? Per the law, he's perfectly legitimate in putting his own unique serial markings on the lower, and designating it for either pistol or rifle use. But it's his responsibility to declare it for it's use. Smart money would be to designate it as a pistol lower, and then, if so desired, the gun could be converted to rifle configuration and back again without hassles.
Looking at the Remington XP-100, it looks vaguely familiar, doesn't it - especially the repeater version. Where have you seen that bolt action before? A-Ha! The Remington Model 7 rifle!
Right down to parts interchangeability, they're assembly line cousins. But the serial numbers and roll markings on the receivers are coded/designated separately for the XP-100 pistol and Model 7 Rifle.
Now, once you get into AOW's or SBR's, it's a different ball of wax, but that receiver still started life, if you didn't make or finish construction of it yourself, as a designated pistol or rifle.