I have a strong interest in clad coinage. I was reading about the 1982 no P dime and began to wonder how we know that they came from Philadelphia. A Denver-minted dime with no mintmark would look the same. The only reason we know about 1922 plain cent is that none were minted in Philadelphia. Is the P designation based on the release area being in a part of the country that normally would get primarily P mint coins, or is there another reason?
I believe you are correct, the area they were discovered, determined what mint they were from. Here's a link to an article. http://www.pcgs.com/News/The-1982-No-P-Mintmark-Roosevelt-Dime
I assume that many mint marks were left off of coinage over the years, but if the Philadelphia mint produced coins that year, there would be no way to tell the difference.
Many of the '82-NMM dimes were found in bags of Philly dimes that had been returned to the banks at the end of the seasin by Cedar Point Amusement in Sandusky Ohio. All of the released dimes can trace at least a few back to Philly bags.
