*squints trying to identify characters* ...Noooo, but that's the wrong way round! David is a foot taller than Alan in the book! :D

And a third theory about David's age, huh. I suppose if Stevenson (or anyone else) got confused about moving the historical action from 1752 (when the Appin Murder really took place) to 1751, that could cause errors of a year, though you'd expect it to be in the opposite direction.