That means it was one of a very limited number of guitars made to a new design. If it was, say, a J-50 prototype, it might be worth a wad -- or not. If it was a model that never went into production, it might be even more -- or not. It might even have a "history," as some prototypes are loaned to well-known players for evaluation. Only someone knowledgable about collectable instruments would be able to say. It might be worthless as a playing instrument and still monetarily valuable, or the other way around. It's a funny business.
But it's a cool find no matter what the value. Did you get to play it? What was it like?
-HM