The phrase "would have been" in the sentence you provided is not being used to express the subjunctive mood. Instead, it is used to speculate about a hypothetical past situation. In this context, the author is suggesting that the unique piece being discussed was likely made by one of the best craftsmen in the Anglo-Saxon world. By using "would have been," the author acknowledges that they are making an inference or assumption about the craftsmanship of the piece based on its uniqueness and quality. The missing "if" clause in this sentence could be something like: "If we could trace its origins, we would likely find that this unique piece was made by one of the best craftsmen in the Anglo-Saxon world." or "If we had more information, we would probably discover that this unique piece was made by one of the best craftsmen in the Anglo-Saxon world." By using the conditional "if" clause, the author would be implying that the statement about the craftsmanship being made by one of the best craftsmen is based on a hypothetical scenario or an assumption due to the lack of definitive evidence or knowledge.

There is no subjunctive mood here. To understand the sentence, let's first look at it with "would" removed. Then we'll add "would" back and see how that changes things. Without "would" (and a few extra words), the sentence is: "this piece has been made by one of the best craftsmen..." This is present perfect tense, passive voice, and makes a straightforward statement about the creator of the piece. It is equivalent to this sentence in active voice: "one of the best craftsmen has made this piece". You can insert "would" into both of these sentences and the only thing that changes is that uncertainty is introduced. Rather than stating facts, the two sentences now state an inference. They express probability rather than certainty: "this piece would have been made by one of the best craftsmen..." "one of the best craftsmen would have made this piece".