1964 proof quarter.... Error?

This coin was in an original 1964 proof set. The whole set was submitted for grading. The original owner thought it might be an off metal. In hand it has the same appearance as my quarters that are struck on cents. What do you think? Has anyone encountered toning of this sort on proof sets. Also I thought I would mention the other coins are pristine with no strange toning.

WTB... errors, New Orleans gold, and circulated 20th key date coins!
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Comments
Looks like a BS from here......???
Can you take pics of the whole slab and reverse?
Too many positive BST transactions with too many members to list.
See http://www.doubledimes.com for a free online reference for US twenty-cent pieces
<< <i>Also the reeding on the edge is all copper. Could it be possible that it is struck on a quarter sized copper blank? >>
NO.
<< <i>Not an off metal, and not a broadstrike as suggested. The coin is as struck and is not an error at all. What you have is what PCGS called it...Artificial Toning. I have seen proof sets from the 1960s with the same look. Generally, the package will have cracks, which will allow the coin to tarnish is not stored properly. >>
Many of these are called "Questionable Color" due to their rather hideous appearance. It is
a tough call with many, since they are naturally toned, not artificial. I see a lot of the spotty
brown toned pieces, often still sealed in the Mint plastic.
Definitely not an error.
I firmly believe in numismatics as the world's greatest hobby, but recognize that this is a luxury and without collectors, we can all spend/melt our collections/inventories.
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