Here's another list of "firsts" within the US commemorative coin series - some a bit esoteric. As before, nothing "previously unknown," just the result of taking a closer look at the coins. Enjoy!
Design Element FirstsPeople
First coin to portray a President already depicted on a circulating US coin: 1918 Illinois Statehood Centennial half dollar / Abraham Lincoln's portrait on obverse - followed portrayal of Lincoln on US cent (first issued in 1909)
The 1982 George Washington Half Dollar is the first coin in the modern series with such a distinction, it features an equestrian portrait of George Washington - Washington first appeared on the circulating
Washington quarter in 1932.
(Note: I consider the 1932 issue to be a circulating commemorative - see1932 Washington Quarter for more.) The 1990 Dwight David Eisenhower Silver Dollar was the first of its denomination to meet such criteria. Its portrait of Eisenhower follows use of Ike's portrait on the US Dollar of 1971-80.
The 1997 Franklin Delano Roosevelt Half Eagle is the first Gold coin in the modern series with such a distinction, it features a depiction of a right-facing FDR (while on a US Navy ship) and follows the
Roosevelt dime that was first issued in 1946.
A case can be made for the 1975-76 Bicentennial coinage as being the first coins within the modern series to meet this criteria as each of these circulating commemorative coins features an obverse Presidential portrait in use on a previous regular-issue circulation coin.

First commemorative coin to feature a portrait of a non-President previously in use on a circulating US coin: 2006 Benjamin Franklin Silver Dollars - Franklin first appeared on the 1948 Half Dollar .

First person to appear on more than one commemorative coin: William McKinley - 1903 Louisiana Purchase Exposition Gold Dollar and 1916-17 McKinley Memorial Gold Dollar.

First President to appear on coinage of a US Territory: Franklin Delano Roosevelt - 1935 commemorative One Peso for US Territory of the Philippines.
FDR might be the only President to ever have this distinction, as current US policy has its territories using regular
US coins and paper money vs. Territory-specific issues.
Inscriptions
First coin to feature a Latin inscription (other than E PLURIBUS UNUM): 1920 Maine Statehood Centennial half dollar - "DIRIGO" = "I Direct" (found at top of Maine State Seal on obverse)

First coin to feature an Italian inscription: 1934 Maryland Tercentenary half dollar - "FATTI MASCHII PAROLE FEMINE" (traditionally translated as "Manly deeds, womanly words") but which Maryland officially translates as "STRONG DEEDS, GENTLE WORDS".
Buildings
First coin to feature a building as part of its design: 1916-17 William McKinley Memorial Gold Dollar - it features the Memorial Building constructed in his honor in Niles, Ohio.
The 1922 Grant Centenary Half Dollar is the first Silver commemorative coin with the distinction - it features Grant's birthplace house in Point Pleasant, Ohio.
The 1982 George Washington Half Dollar is the first coin of the modern commemorative series to feature a building - Washington's Mt. Vernon home on the reverse.
A case can also be made for the 1975-76 US Bicentennial Half Dollar as being the first in the modern series - as a circulating commemorative, it features Independence Hall on its reverse.
Designers
First commemorative coin to officially feature a design from an artist/sculptor outside of the US Mint: 1915 Panama-Pacific International Exposition Gold Dollar (Charles Keck) and 1915 Panama-Pacific International Exposition Gold $50 coins (Robert Aitken) - a tie.
Government Jurisdictions
First commemorative coin to honor a US State: 1918 Illinois Statehood Centennial.

First commemorative coin to celebrate a US County: 1936 York County, Maine Tercentenary.

First commemorative coin to recognize a US Town/City: 1935 Hudson, NY Sesquicentennial.
If you'd like to review the previous "Firsts" posts, check out:
-
Commemorative Firsts - Part I-
Commemorative Firsts - Part IIFor more of my topics on commemorative coins and medals, see:
Commems Collection.