This is a Standard Two-Light Storm Sash. This type of sash is used on the exterior side of the primary window.  The two-light storm sash has been around for over 125 years and is the traditional way to make your original windows more energy efficient by creating a dead-air space between the storm sash and the primary window.  Unlike a modern dual-pane window, however, the traditional storm sash provides the added benefit of protecting your primary window from the elements.