Is it to know by what means an immensely aged man could be identified as Napoleon if we discovered that, amazingly, the Emperor had not died when we supposed, but had lived until the present? [...] Surely not. To understand the name 'Napoleon', one must know what the correct way, or at least a correct way, was to identify someone as Napoleon when he was alive.This strikes me as wildly implausible. Not only is it doubtful that most of us could indeed, as a time traveler to the 18th or early 19th century, recognize Napoleon on the street, say, in a group of soldiers. (My idea of his visual appearance has its origins in a couple of paintings and a few movies. It's probably not even good enough to identify those actors who played him on the street; much less the man himself, who in all probability only has some degree of resemblance with them.)
And even if we could, normally we wouldn't say that that is what it means to know who Napoleon was. To understand the name, at present day, is first of all to understand that it is the name of a historical person, someone important enough that many people have heard about him. We would have to know some basic historical facts as well, although there might be no specific set of facts that would count as necessary and/or sufficient: if someone thought Napoleon was a Spanish painter in the early 20th century, we would say that person doesn't know who Napoleon was. If he says something like: "Oh, I know, it was this French king who made some conquests in Europe a couple of centuries ago", this we might count as knowing (even though we know Napoleon wasn't a king, but an emperor).
In short, in order to understand this name, we would have to find our way round in today's established history, not in eighteenth century Paris's palaces. Contrary to what Dummett seems to think, we wouldn't have to put ourselves in the shoes of some past observer in order to know what the name 'Napoleon' means.
 Michael Dummett, Thought and Reality. Oxford: Clarendon 2006, 74–75.