As I look closely at the condition of your "mustard/error" HS chip, I can see the clay composite itself has some definite issues. If I would have to speculate, I would say the chip appears to have been subjected to a harsh chemical or cleaning agent resulting in the degradation of the clay. Even on the scan you show, I can easily see both mottling and pitting of the clay. I would also logically speculate this same cleaner affected the gold hot stamp as well.

Using a harsh cleaner or a fairly mild cleaner with the chip was left soaking too long could result in such a condition. Clearly with the damage to the clay chip itself, there would be no way I would represent this chip as an error. If the integrity of the clay was pristine, then yoou might have some hope. However, that is not the case here.

A research experiment would be easy enough to do on common current similar 25 cent chip. Should the results conclude a confirmation of your speculation, the experiment could be duplicated by a non biased third party to put everyone at ease.

I would highly recommend doing a the experiment before deriving at either of your two conclusions.

Doing such an experiment would not only put every one's scepticism at rest, but it would also allow a buyer to buy with confidence as welll as possible having a variation added to the TCR.