It is a matter of logic.
It has to be something added to the surface as a clear portion of the last digit and the D mint mark is visible.
Next, you can still see most of the rim. The rim acted as a barrier to keep whatever that foreign material is from spilling over the edge.
It can't be a giant "
Cud" error which I'm sure some people will try to say it is because the rim is still visible.
If the die broke forming a
Cud, the rim would have broken off as well.
The only logical conclusion is foreign material added to the surface of the coin. What it is cant be determined from a picture. The fact that it looks like the same material is purely coincidental.
There is nothing in the Minting process that can cause this.
It can't be a split planchet error as the coin would be thinner and the mint mark would likely be missing.
PS: I've seen this kind of thing before:-)
Thanks,
Bill