Thanks for the reply's, guys. I doubt that my car was an export (and I don't even know what the tag would look like) but I was somewhat puzzled because the bolts had been carefully sealed over. It would explain why I never found a build sheet, though, since I obviously wasn't the first one to pull the back seat. I've had my gas tank out during this rebuild and didn't find anything tucked up there, either.
If I can locate some correctly dated belts I'll tuck them away somewhere for posterity, but I do like the "clean look" of the rear seat, sans belts