What this coin looks like (obverse, reverse, mint mark location, special features, etc.): One major variety of the 1842 O Seated Liberty Half Dollar features the reverse of 1842 which has large letters and also a medium date on the obverse of the coin.
1842 was a transitional year for the designs of both the obverse and reverse of the seated liberty halve. The rarest and most valuable varieties feature the older 1839 design on the reverse of the coin. This design features much smaller letters in the words "UNITED STATES OF AMERICA" and "HALF DOL.". Later on that year: The newer 1842 O reverse was introduced and included much larger letters. The difference between the Small Letters vs Large Letters is depicted below in the example comparison image:
There was also a change in the style and size of the date on the obverse of the coin. Earlier coins had a smaller date with smaller numbers whereas the newer design featured larger numbers in the date (known as Medium Date). The difference between the Small Date vs Medium Date is depicted below in the example comparison image of the coins:
Coin valuation chart: Typical coin prices, values and worth per grade or condition - in USD