As to belonging to the castaways I could have added that this would exclude salting the island with items sourced from elsewhere to falsify a proof - but as I cannot conceive why anyone would do such a dishonest thing then I felt that it needed to be left unsaid as irrelevant.
It's completely relevant.
All readers of archaeologists have to trust the word of the diggers about what they found where they found it. Once an artifact is moved out of its original context, only the word of the digger remains. There is huge fame and money to be made by counterfeiting discoveries (Piltdown Man, Cardiff Giant, Ninov's falsification of data for element 118), etc.
Of course, this is a two-edged sword. If it is an anxiety about TIGHAR's work, however tastefully, modestly, and humbly stated, it applies to the New Britain hypothesis, too.
Passing over things that don't need to be said in silence is such a beautiful rhetorical ploy. So, for example, I do not question your credentials as a Ph.D. in archaeology, because I cannot conceive why you would puff yourself up as something you are not. I take you at your word, without proof, because that is the polite and respectful thing to do.
In other words, I am operating by faith, not by proof. I believe it is reasonable to do so. If I was the skeptical type, I'd ask for proof of your claims about your status--proof that could not be counterfeited, proof that is purely objective, and proof that does not rely on taking anyone's word about how they know that you are who you say you are and that you possess the credentials you claim to have.