Hi Val!
I think your understanding of the "This is of great significance" language is correct--it introduces a conclusion.
That language is implicit in Ben's breakdown of the conclusion in the prior post: "Conclusion: [This is of great significance because it shows that] The B-36 Bible was printed by Gutenberg and the Vinland Map is really from the 15th century."
You're also right in your process of assessing whether or not it is a conclusion, by asking whether it is used in support of other statements or instead is supported by other statements. Here, you correctly point out that a certain finding's being "significant" is what other statements in the stimulus support (it is presumably significant because of the implications about Gutenberg and the Vinland Map).
With that conclusion in mind, one can see why there is a flaw in the reasoning, identified in answer choice (A): "the results of the analysis are interpreted as indicating that the use of titanium as an ingredient in fifteenth-century ink both was, and was not, extremely restricted." Part of author's conclusion about the significance of the findings is that the author understands them to imply that B-36 was created by Gutenberg--i.e., since the Gutenberg Bible contained titanium it assumed that another source with titanium must have also been created by Gutenberg. This assumes that titanium was especially restricted, such that one could know that a book's author was Gutenberg because it contained titanium (titanium
Gutenberg).
At the same time, it takes the findings to mean that the existence of titanium in another object--the Vinland Map--to support its authenticity. This second assumption is in tension with the first one. That is, perhaps the findings do indeed show that titanium was indeed used before the 16th century, earlier than previously thought, with the findings of titanium in the ink of the Gutenberg Bible supporting this. However, if titanium use in ink really was limited to Gutenberg, then this undercuts the attempt to argue that its use was more widespread and extended to the Vinland Map.
Perhaps that explanation is unnecessary--again to your concern with identifying the conclusion, it certainly seems that you looked in the right place!