One thing, and I'm no expert, but I understand the sun is considered a "grade sensitive" area, which might account for some of the lower guesses. Can you tell if the somewhat horizontal lines on the sun are post mint contact marks or something else? The coin does look nice, I was being conservative.
I think there are actual scuff marks on the body of the sun, near the top, and the lines in between the rays at the top are die polish again. Major marks for me are on the calf, hip and neck, but again, only with great magnification are they noticeable. No worries on the grading, I'm equally fallible
First, congrats on the grade! I had guessed 65, based on how I interpreted the luster. As for Full Thumb - this designation is not recognized or labelled by any TPG, but Walker collectors absolutely use this amongst themselves. Bruce Fox mentions it in the book he wrote about Walkers in the 90's. The thumb is the highest point on the central design - if you look at enough walkers, you'll start to realize that having a fully thumb is actually pretty scarce especially on certain dates.
Well I'm very happy to have done this little GTG, having gleaned info that I certainly wouldn't have if not. @physics-fan3.14 Many thanks for the fascinating input! Any other keys to collecting this series?