The BC hasn't had a formal studbook very long compared to some breeds, and there's giant holes where it's really easy to run into a "NR" dead end - even just a few generations ago.
I have two dogs whose ancestors were mostly hill-bred, though their near relations were National and Supreme Champions (ie, parents or grandparents). That's the way it often works.
My pup, however, is from really old, famous lines (for BCs). Oddly, although I'm a real freak for pedigrees and love to play the pedigree tracing game, I didn't do it when I purchased Ted. It wasn't until his papers arrived from ABCA (which include a five generation ped), that I sat down and started going back from the sixth and on - he's got several lines that are traceable back to the foundation dogs of the breed. My old Ben dog is similiarly bred and that is true of him too.
You can get back to Old Hemp, Loos II, the Caps, etc, on most working BCs, pretty easily, first because the documentation is there, and second because in terms of dog generations, many of them aren't that far away. Working BCs tend to be bred late in life compared to show dogs - first litters not until three or four on males, sometimes later on bitches. I had one dog that went back to the late sixties (the time of the famous Wiston Cap, but he had no Cap himself) in only three generations - that's darn close to human generations.