Another version I read was the distributor was wondering how to move some slow selling Ginger Beer and the bar already had the copper mugs, the Smirnnoff's guy suggested Vodka because of course, he was the Smirnoff's Rep. And the drink was born out of utility, not all creativity.

Also interesting is the recipe I remember is lime not lemon? Hmmm? Now I wonder...

1.5 oz SMIRNOFF® No. 21 Vodka
4 oz Ginger Beer
Fresh Lime
Serve in a copper mug.

And yes the copper mug does make a difference, kind of a tingly addition to the taste.