What this coin looks like (obverse, reverse, mint mark location, special features, etc.):
There are two major varieties of the 1873 $20 Liberty Head Gold Double Eagle. One variety is known as the "Open 3" - which features wider spacing between the top and bottom ends of the number "3" in the 1873 date - as depicted above. The "Closed 3" variety has narrow spacing between the "3". The difference between the 1873 Closed 3 vs Open 3 $20 Double Eagle is depicted below by the red spacing lines in the example comparison image of the coins:
Coin valuation chart: Typical coin prices, values and worth per grade or condition - in USD