Well.......it might be a worn down die number. But more likely it's just a smudge of some sort. Very difficult to know. f it is a die No, what a shame it's worn down. Of course, they're as rare as hen's teeth, and you hardly ever see them for sale, even at the top auctions. Alfnail had a worn one up on e bay for £995 ( think Die No 4), but he withdrew and sold to a previous client. One of my biggest wants, an 1863 die No - not fussy as to which, or condition of specimen. It would have been a massive shame for sure, and after researching more about them to try and assign my coin I can absolutely confirm what you're saying, very very hard to find examples to compare to. Alfnail always has something for sale that's worth drooling over haha