OPAL, as printed in The Old Waldorf-Astoria Bar Book, 1935, a collection of Pre-Prohibition, Classic Cocktails.
Opal
Add to iced tumbler, shake and strain into a cocktail glass:
1 dash of orange bitters
1/2 Plymouth Gin
1/2 French vermouth
Stir. Add 1 dash of Absinthe on top.
The Old Waldorf-Astoria Bar Book