....what guide should include it? Should it be in a Nevada guide? A Colorado guide? An Atlantic City guide? I would suggest that if it is truly "unknown" it shouldn't be in any guide unless or until someone publishes a guide to unknown chips.

If a collector has a reason to believe that a chip may have been used in a particular casino in a particular location, but doesn't have proof, I see nothing wrong with "attributed to" until its actual use has either been proved or disproved.