I have a book ‘The Cocktail Bar’ from 1977 by ‘Carlos’ of Raffles which gives the recipe as 1/6 Gill (25 ml?) each of ginger ale, Benedictine and Scotch whisky rather then bourbon- I have to say I prefer the Diffords but like the simplicity of Carlos!
Sadly, I don't have a copy of Carlos of Raffles book, but I do have Jones' Bar Guide from the same year. This cocktail also features in 1930 The Savoy Cocktail Book. I've added notes from both books above. I’ve just tried with bourbon and scotch and prefer the latter as drier, so I've amended my recipe accordingly. Many thanks for your feedback.