Just suppose for a moment that it's fake and the error did not occur at the mint. Someone may have doctored the '8' to look like a '5', that is how it looks to me. If I would want to create another 'full' number out of an existing number I would take the one with the most meat, I think the '8' would be the ideal candidate and a '5' would fit perfectly over the top in a hollow punch.
Spink mentions this as 8/5 as being Extremely rare, and conventionally, the first digit predominates over the second. This is a 5/8.
Just found the slab photograph at Heritage which looks even more suspect don't you think?
It would need RM (not PCGS, sorry PCGS) authentication before I coughed up over $4,000 for it.