Sounds like two different things. If this BET is the only one, it was likely a flake of pulp that came off after the litho printing. It might be considered an error, but I doubt it would have much of a premium. On the other hand, if that's how it come off the press, I'd still call it an UNC.
If there are a number of notes, as Gary suggests, then the pulp was likely stuck on the litho "offset" roller. I'd still call it an UNC. (The "every other part" is a bit strange, but there are several discussions about that already).
