Philosophical zombies

[The Grand Nudger]

If they have meaningfully different brains then we would likely refer to that meaningful difference as a possible explanation between the two.  If whatever meaningful difference they have produces no discernible effect..such as we cannot otherwise tell between the two which is the control and who is the outlier....then we might suppose that specific disparity between us is functionally inconsequential to consciousness in the first place. 

I think the problem with that view is that it ignores the possibility that the difference is very much to do with the difference in consciousness... and despite the fact you can tell that the difference is the explanation for the difference in consciousness.... the vital point is [i]you still don't know [u]which one is conscious or why that one and not the other. And is that not a possible hard problem?

There is only -the- hard problem.  The hard problem of consciousness is the notion that consciousness is not (or may not be) explicable by reference to a functional definition of the operation of a physical system.

What you are discussing is considered a soft problem. Determining which physical system, or which parts of two comparable systems, are the relevant bits to consciousness. Between us, and between those who seek to explain consciousness, a "difference in consciousness" is explained by a difference in the system that produces it.  Individual brains, individual consciousness.  If it is understood that, broadly, brain type x produces consciousness as we know it, that we are all meaningfully type x..but that one subject is type y.  Either type y is also conscious..as n they are both conscious, type x is not conscious (and the rest of us are not conscious), the very difference between x and y describes and defines which of the two would be conscious and unconscious, or the specified difference is immaterial to consciousness.  

Okay so you wouldn't consider such a person a P-Zed because P-Zeds are specifically postulated in an attempt to argue against physicalism. Is that a fair summary of what you just said, for you?

P-zeds are designed specifically to have no physical difference between anyone else in comparison.  Not that we're unaware of what it is, that there literally isn't one.  Identification and classification are soft problems.  The objection they're meant to express (whether used to argue against physicalism or not) doesn't work otherwise...and it's useful to remember that it exists only by fiat in the thought experiment.  

And still, what would you call a person who isn't a P-Zed but has no consciousness and yet behaves exactly the same as we do, despite having a different brain, but not different enough for us to not wrongly dismiss it as normal brain variation?

If they have normal brain variation we would expect normal brain function.  If they do not, we do not (we're sometimes surprised on this count, ofc).  If the "disqualifying" variation falls below some detection threshold then you are no longer referring to a brain with a discernible difference....but, instead, a brain that at least seems as normal as any other.  Nevertheless, the difference does exist. So, if we were deadset on declaring one concsious and the other notm or in fact knew (somehow - such as a fiat declaration in a thought experiment) that one was and one wasn't..we'd likely point to the non-heterodox brain as the odd man out.

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And the point is the physical difference is that the person isn't conscious.... but we are unable to tell they're not. So they're not a P-zombie, but what are they?

A behaviorally convincing mimic existing in a world without a functional definition of consciousness? Probably don;t have to be a p-zed or anything like it to qualify for that title, either. A chatbot could already qualify. If it fools anyone, then it;s met the relevant metrics for consideration in your proposition despite having no brain (or body) whatsoever, let alone a similar one that would be expected to perform a similar range of function anyway.