Please give me your opinion of this ‘64 D quarter, in regards to possible doubling, especially the mintmark.
The Mint Mark cannot be a Doubled Die because it was hand punched into the Die. What it can be is a RPM.. Repunched Mint Mark.
Check this page for markers on your coin. some places in the pics I cant see to verify known specimen. https://doubleddie.com/1949204.html
Re the mint mark check it against these listings WRPM-006 and WRPM-007 http://doubleddie.com/1237766.html