This isn't "post mint damage", actually its from the edge lettering on the coin. If you look you will see that each letter causes a thicker edge at the rim of the coin. The lettering is applied to the blank prior to striking. The obverse and reverse image is being fully struck but there isn't sufficient pressure to force the coin's metal fully fill the die to form a full rim. Sadly this causes the coins to look poorly made and amateurish.