The most obvious answer to the question is contained in the label. It was distilled in 1942. It was bottled and labeled after that. No prescience required.
As for the 1942 date, since the war started in 1941, I would assume that what they really meant was "distilled before the imposition of wartime prohibition."
And that said, I'm not so sure that Wikipedia entry is exactly correct. I don't believe whiskey distillation was so much banned as it was impractical due to restrictions on raw materials and the demands for wartime production of neutral spirits. I know that some whiskey was produced during the war because the services required it "for medicinal purposes." The amount produced was simply way short of demand.
My further assumption is that this labeling was both an indication of age, as soad said, as well as a way of stating that no wartime restrictions were violated to make it.