Stir, strain into a coupe, olive garnish.
So... an improved dry Martini. Yes, it's small. The Savoy has this as 1/2 Plymouth and 1/2 dry vermouth, so feel free to play with the volume. The Waldorf Astoria Bar Book (1935) has this as 2/3 Holland Gin (Genever) and 1/3 sweet vermouth, with a dash of Angostura bitters, which is weird - they didn't get it from Duffy.
Harry Johnson's Bartender's Manual, 1934 ed. pg. 268