As with many items it's basically popularity and just rumors of an extinct mint or something like this. The S mint used to make normal, regular, everyday coins just like the other mints. Then not long ago they started to specialize in Proof coins only. So people started to think that any coin with an S mint would someday be worth a massive amount of money so they started to hoard them. As the rumors of a future great investment in any S minted coins continued to spread, more and more and more people would tell others of the coming of the great increase in any S minted coins.
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I have noticed a few people on here talking about pulling out any "S" mint cents for their collection.
And it's more than just a few people, it's a lot of people. Many of the really older S minted coins are worth more but many are really not but the more that save all S minted coins, the less there are out there so yes they will be worth more. MAYBE.