To answer the questions , that have already been answered. The marks on the case are from the pinion nut backing off like has already been stated. It is very common to see on a lot of 8 3/4 sure grip units for some reason.
It does not hurt the use of the unit. Just a cosmetic thing.
I have not had it apart. It is a used core unit and priced according. Not a new part a used 45 year old part.
As of now it is Sold pending payment if that deal falls through there are several others in line for it.
Matt