What kind of discoloration is this and how would it affect its grade? 0

[gotpong]

Hello everyone, just curious if anyone has ideas on what kind of discoloration this is on the back cover of this book and how would it affect the grade?  It seems to be the inverse of a dust/sun shadow but being relatively new I'm not sure and could use some opinions from people who are smarter than me.  Thanks in advance.

[The Lions Den]

That's really mild, my friend, especially for a book from that era (I'm guessing 1965-1966). I'm a high grade collector, and that amount of discoloration doesn't bother me whatsoever. In fact, that's the kind of back cover I dream about. There may be other flaws on the book, but this slight flaw shouldn't keep the book out of Near Mint condition... 

Edited May 6, 2018 by The Lions Den
Additional info added