There’s no information on the Upstairs Cocktail in The Savoy Cocktail Book, recipe aside, but mixing up a couple of these reveals all you need to know: they are a refreshing and low-proof cocktail before dinner.
Combine the Dubonnet and lemon juice in a collins glass with ice, then top off with the seltzer. Stir to mix. Express the lemon zest over the drink and drop it into the glass.