About 200 years later in the 12 th century, this parchment manual script became a palimpsest when a scribe used the parchment to make a prayer book. So the pages, the pieces of parchment themselves, had been preserved. But the Archimedes' text was erased and written over, and no one knew it existed. It wasn't until 1906 that a scholar came across the prayer book in a library and realized it was a palimpsest, and that the underlying layer of texts could only have come from Archimedes. That was when his work The Method was discovered for the first time . 

请问老师，这里的could have done是情态动词表推测的哪种用法？