This scar was the only hint of Harry’s very mysterious past, of the reason he had been left on the Dursleys’ doorstep eleven years before. [The situation is that Harry, the main character of this novel, has a scar on his forehead that makes him a special wizard because it is to do with Harry's very mysterious past and because of the past event, he lost his parents and got the scar, so he lives with the Dursley family.]
To me, "of the reason...before" seems to have been used as a non-essential prepositional phrase, so I think "of the reason..before" is connected to "Harry's very mysterious past". Though, there were some native English speakers who answered me that it's connected to "the only hint", but I don't think the answer is right, because the interpretation doesn't sound natural if it's really connected to "the only hint". I want to know to what the prepositional phrase is connected.
(And as a similar structure, "...using fiction as a mirror image of the past, of the historical time", here, "of the historical time" seems to be the non-essentical prepositional phrase connected to "the past", not "a mirror image".)