I have never heard about a 15 cent coin or even a proposal for one. However, cents struck with dime planchets are sometimes referred to as "11 cent coins", so I suppose if a nickel was struck on a dime planchet that that might be referred to as a "15 cent coin".
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Obscurum per obscurius
<< <i>It would be too small. The three cent silver is the smallest US coin. >>
The 1792 Silver Center Cent was an attempt to get silver into a convienent sized coin.
to turn over with my nose. Its very hard for bears to turn coins over you know.
Camelot
And the three cent silver is the second smallest US coin. The Type I one dollar gold is smaller in diameter by 1/2 millimeter (but slightly heavier).