From the book:

If you ever read this tale, you will likely ask yourself more questions than I should care to answer: as for instance how the Appin murder has come to fall in the year 1751, how the Torran rocks have crept so near to Earraid, or why the printed trial is silent as to all that touches David Balfour. These are nuts beyond my ability to crack. But if you tried me on the point of Alan's guilt or innocence, I think I could defend the reading of the text. To this day you will find the tradition of Appin clear in Alan's favour. If you inquire, you may even hear that the descendants of "the other man" who fired the shot are in the country to this day. But that other man's name, inquire as you please, you shall not hear; for the Highlander values a secret for itself and for the congenial exercise of keeping it I might go on for long to justify one point and own another indefensible; it is more honest to confess at once how little I am touched by the desire of accuracy. This is no furniture for the scholar's library, but a book for the winter evening school-room when the tasks are over and the hour for bed draws near; and honest Alan, who was a grim old fire-eater in his day has in this new avatar no more desperate purpose than to steal some young gentleman's attention from his Ovid, carry him awhile into the Highlands and the last century, and pack him to bed with some engaging images to mingle with his dreams.