The Side-car is first mentioned in two books from 1922; both of which state that it is the invention of one person, MacGarry, working out of the Bucks Club, in London, England.
Cocktails: How to Mix Them, by Robert Vermiere, 1922.
Note: the recipe above is basically "equal parts of each".
For those who are interested, a Gill is equivalent to a quarter of a British pint, and that is 142 millilitres. The Side-car recipe above is, therefore, 71 millilitres in total; Not too far from the 75ml drink you would expect in a UK bar.
The other of the two 1922 Side-car cocktail mentions is from Harry MacElhones' "ABC of Mixing Cocktails":
"Recipe by MacGarry, the Popular bar-tender at Buck's Club, London."
Harry's actual recipe is the same as Robert Vermiere, listed above.
After the first mentions of the Side-car in 1922, it is not until 1930 that it is to be found again in a cocktail book, namely the Savoy Cocktail Book, by Harry Craddock. Craddock lists a recipe that is 2 parts brandy, 1 part Cointreau, and 1 part Fresh Lemon Juice. And so, based purely on this 1930 citation from the Savoy, in London, it is assumed by some that this exemplifies an "English School" of Side-car recipes. However, in 1923, a newspaper journalist situated in Paris, referred to the Side-car cocktail in their correspondences back to America:
"Another new cocktail, second only in popularity to the monkey gland, has been named a "side-car," because it takes the imbiber for a ride. Two-thirds brandy, one-sixth Cointreau and one-sixth lemon juice make up this concoction."
This excerpt shows that a much drier version of the Side-car than the one contained within the pages of the Savoy Cocktail Book was being imbibed, in the very next year after the publications of both MacElhone and Vermieres books.
There is no English School of Side-cars, it was just a feeble attempt to explain why two recipes were different in two books, when there were no other mentions of the drink in the intervening eight years.
Some people prepared their cocktail glasses with a sugar lip, when concocting side-cars; This sugar-lip on the glass dates back only as far as 1934; and is not an original feature of the Side-car. However it was the way I was taught to make a Side-car, and is the way I drink them, if I am brave enough to order one.
Whichever way you make your Side-car make sure to use first-rate ingredients; namely freshly squeezed lemon juice, Cointreau (or some other good quality Triple Sec), and a decent Cognac.