A square whiskey. In a square bottle. In this case, “Square” meant good, honest, fair - a square deal, for example. Or, it was marketed at the Borg demographic. No, they’d probably prefer a blend.

This had no relationship to Mount Vernon, or George Washington, although the suggestion that it was bottled in bon "under supervision of the U. S. Gov't" gives it a little taste of ol' Cincinnatus.