Unlisted RPM? Might be worth checking your 28-S dimes.
![Shurke](https://us.v-cdn.net/6027503/uploads/userpics/BIYRXW1OV7UX/nPDY9O18LE4UD.jpg)
While perusing eBay listings, I came across this 1928-S dime that appears to have a strong RPM (which isn't listed at either Variety Vista or at Wexler's site). If it is indeed an RPM, it would be among the best in the Mercury dime series. It might be worth checking your 1928-S dimes to see if it appears on any of your coins.
Here's an enlargement of the mint mark on the eBay coin:
And here are the full obverse and reverse shots:
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Comments
Looked up a photo of the same coin from a previous Heritage listing; it looks like what you're seeing is die deterioration doubling:
I didn’t think to check the Heritage archives. Thanks for doing that legwork.