There are a lot of alcohol combinations which now became classic. These combinations are so good that you’re always reverting to it and think that you do need any others.
Whiskey (and whisky too) with sweet vermouth create such combination, a wonderful duet, an exceptional base for further experiments. Well-known Manhattan (with bourbon or rye) or Rob Roy and Bobby Burns Cocktail (with scotch), it all are just three great cocktails among dozens of other good ones in which whiskey mixes with sweet vermouth.
The Forty-Five recipe has been found at Drambuie American promo-website. Another place where the cocktail was mentioned is The Intoxicologist blog. The article about cocktails based on Drambuie liqueur is awesome as well as most of other posts there.