Leap Year Cocktail

For Christmas this year, my sweetheart got me the Savoy Cocktail Book, so I think the next several daily recipes will be adaptations from that source. Here is one, listed with the following passage:

"This Cocktail was created by Harry Craddock for the Leap Year celebrations at the Savoy Hotel, London, on February 29th, 1928. It is said to have been responsible for more proposals than any other cocktail that has ever been mixed."

Here's my version:

Ingredients

2 oz. Gin
1/4 oz. Grand Marnier
1/4 oz. Sweet Vermouth
1/4 oz. Sour Mix.
Lemon Twist, for Garnish

Instructions

Fill a Boston Shaker with Ice
Pour Gin and Sour Mix
Pour Grand Marnier and Sweet Vermouth
Shake and strain into a chilled cocktail glass
Garnish with a twist of lemon