Looking at recipes on the Internet it seems eggnog barely contains any whiskey by volume. By comparison German Eierlikör consists mostly of korn (a 32% ABV clear liquor similar to vodka) or white rum.

I'm not sure what 1950s eggnog looked like but I'm guessing it was more like the German style Eierlikör (i.e. made mostly with liquor and eggs) than the modern eggnog (i.e. mostly milk and cream).