OK. Most likely a filled die error. Some non-compressable material, grease of such, gets stuck in the digit, which is a recess on the die, so it looks smooth on the coin.
There is a lot of impact damage on the coin as well, so it could be debated.
There have been a number of threads in this forum about this type of error. sometimes it is just in one digit or letter like this, sometimes a group of letters are filled.
Jamais l'or n'a perdu la plus petite occasion de se montrer stupide. -Balzac