From the Super Mario Wiki
The e-Coins of World-e.

e-Coins are rare types of coins found only in World-e of Super Mario Advance 4: Super Mario Bros. 3. Fairly well hidden, only three of the eleven levels released outside of Japan have e-Coins.

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/Luigi before being displayed in the World-e 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 appear to serve no purpose.

e-Coins and levels[edit]