From the Super Mario Wiki, the Mario encyclopedia
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e-Coins are rare types of coins found only in World-e of Super Mario Advance 4: Super Mario Bros. 3. Fairly well hidden, e-Coins were initially found in only three of the eleven levels released outside of Japan.
Each e-Coin features a different image on one side; the other side of each e-Coin shows the e-Reader "e." Each e-Coin can only be collected once by Mario or Luigi before being displayed in the e-Coin Castle, where they reside in individual frames with their level of origin underneath. Any e-Coins within blocks or under grass tufts are replaced by 1-Up Mushrooms.
There are eight frames and eight e-Coins, but they serve no purpose.
e-Coins and levels