Up until 1990, all mint marks were hand punched into working dies. After 1989 the mint mark was applied directly to the master die.
Now the reason for the slightly tilted mint mark was that it was punched in that way. From the beginning, when mint marks were applied to the working ides, you will find a whole array of oddly placed, tilted, doubled, tripled and even at times quadrupled punched mint marks. I forgot to mention inverted also.
While oddly placed or tilted mint marks are kind of fun to look at, they command little to no premium,UNLESS they happened to be in another design element.
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