I must say I have to agree with Coinery - whilst there were many high quality contemporary forgeries, it really does look pretty genuine; don't know offhand what a correct weight should be, though... One other possibility is that it is actually genuine and that some foreign matter got mixed into the molten silver when the blanks were made and it is this that has subsequently made the missing sliver of surface silver fall off during its life, a bit like a lamination flaw. To be absolutely definitive, it would need to be seen in the flesh by someone competent.