Oh, there is also another category of firearms - those that have been removed from the NFA list and put on the C&R list. If you look in The Book, you'll see a bunch of short-barreled rifles, mostly Winchesters, that have been listed by serial number. They're factory SBRs whose owners have petitioned the ATF for removal from NFA status. They're exempt from the NFA.
There are probably other examples, but since I drool over Winchesters, that's what caught my eye.
Well we don't rent pigs and I figure it's better to say it right out front because a man that does like to rent pigs is... he's hard to stop - Gus McCrae