Although standard U.S. coins have stayed consistent for decades, many less-common denominations were discontinued and became obsolete. The original article noted that people interested in rare coins could watch for these historic denominations. 
Bullion
Bullion refers to coins made entirely of gold, silver, or platinum that trade for their metal content rather than numismatic value. The original article reported four types of U.S. bullion coins: platinum eagles ($100), gold eagles ($50), silver eagles ($1), and gold buffaloes ($5, $10, $25, or $50). At the time of publication, palladium eagles ($25) were expected in October 2017. These coins remained legal tender, though their metal was worth far more than face value. The article reported that authorized sellers offered them for purchase at that time.
Pre-1933 Gold CoinsAt the time of original publication, the article described the following:
These rank among the more common rare coins and were minted from 1792 through 1933. Technically, 1933 $20 gold coins were struck, but only a small number survive. Most were melted after FDR signed Executive Order 6102, which made private U.S. gold-coin ownership illegal. Pre-1933 gold coins came in several denominations: the $20 double eagle, $10 eagle, $5 half-eagle, and $2.50 quarter-eagle. These denominations correspond roughly with 1 oz, 1/2 oz, 1/4 oz, and 1/10 oz gold coins.
Three-Dollar Piece
Much like the quarter-eagle, this coin was made from 1854 through 1889 for one practical purpose: allowing businesses to buy 100 stamps in one transaction. Once stamp prices rose beyond three cents apiece, the coin was retired because the three-dollar piece had little remaining use.
Twenty-Cent Piece
This coin holds the doubtful distinction of the shortest period of circulation in U.S. history, having been minted only from 1875 through 1878. Its design aimed to create a roughly equal exchange with the French twenty-franc coin, and it had a similar size and weight. Because quarters were already so common, the twenty-cent piece quickly lost public favor.
Half-Dime
This silver five-cent coin preceded the nickel and was issued from 1792 through 1873. Nickel lobbyists promoted a new half dime made from nickel alloy. Once the replacement nickel appeared in 1866, the half dime disappeared within only a few years.
At the time of publication, readers interested in buying or selling rare coins were invited to consult Atlanta Gold & Coin Buyers. The original article stated that the company offered transparent buying rates and straightforward valuations in person at Midtown Atlanta and Johns Creek offices, or safely through the mail.
