If anybody has David Lange's book, "The Complete Guide to
Buffalo nickels", and especially if you have the "Second Edition" printing as I do, please take a look at the photo they have in there for the 1918-S.
Isn't that a real obvious clash mark under the Indian's chin? Yet under "popular varieties" for that year and mint it doesn't even mention it.
I wonder if an editor or somebody took a shortcut and used an obverse photo that may not have been from the San Francisco mint?
On the page dedicated to the 18-D (not the overdate, just the standard 18-D) it does mention clash marks as an occasional occurence...but not so for the 18-S...