As near as my research tells me, spirits of wine was added to the mixture to provide character and fortitude.
As spirits of wine is somewhat difficult to find stateside, a substitute is said to be any grain alcohol of high proof.
Everclear fits the bill, although I am pondering building a small still and distilling some Merlot or Carbernet.
My inside belief is, as a small amount of spitits of wine was used in the original mixtures, some 2-1/2 ounces, the old masters simply bought a quart of stout brandy, used 2-1/2 ounces in the mixture, and drank the remainder as they rubbed the Purdey Mixture into the walnut. Maybe a shot of brandy each time they rubbed in a little finish.
