What this coin looks like (obverse, reverse, mint mark location, special features, etc.): There are a few major varieties of the 1840 Braided Hair Large Cent (Penny). Two major varieties deal with the size of the 1840 date as one had a small date and another had a larger date. The difference between the Small Date vs Large Date is depicted above in the example comparison image of the coins.
Another major variety is the 1840/18 Overdate error. A small 1840 date was stamped over a larger date which resulted in part of the "18" being doubled. A small portion of the underlying "1" is visible in the upper-left portion of the main "1". In addition: Part of the underlying "8" is seen towards the left and lower-left side of the primary "8" and also within it as depicted by the arrows in the example image below: