The cocktail was made for he first time in 1930 and appeared in Harry Craddock's "The Savoy Cocktail Book" 1930. The 1930 recipe doesn't say anything about the lemon twist garnish, but the one in Gaz Regan's book adds this subtle aromatic dimension
2 ounces Irish whiskey
1 ounce sweet vermouth
Angostura bitters to taste