Did a couple of searches for karawara. Nothing meaningful comes up.
From what I see, it is not typical for a Centrodoras to have a spotted, mottled, or granulated appearance.
Next time ask for that to be put in writing, dated and signed.
Based on what traits? You may be right, I'm just looking to learn. So little is known and so few photos of rivasi are out there.
Plus photos of the subject matter in a plastic bag may be misleading too but that's a separate matter. Bags have curvature which distorts the object being investigated.
So I agree with you. A bet may be appropriate, albeit my bet would be a granulosus. Statistically favored by many, many orders of magnitude.