ALUMINUM FINISH ( MILL COLOR ) IS DISCONTINUED AND LIMITED TO EXISTING STOCK>
The ALUMINUM frames moving forward will ALL be PAINTED WHITE 
C-310 aluminum basement window insert with dual pane glass. 

Fits standard 31 x 20 inch basement window

The Columbia Series 310 aluminum basement window is a insulated glass basement insert.
Maintenance free, energy efficient. 
Easy, 1 minute installation.
Requires no tools for installation; uses high tensile steel spring clips to secure the insert in the opening.

Color: White Aluminum finish standard

Fits in buck with 1/8 inch drop-down (see third image)

Standard Sizes
31 x 12-5/8
31 x 16-5/8
31 x 20-5/8
31 x 36-5/8 egress

Priced and sold each.

Resources: 
Article: How to Replace Basement Window Inserts
Video: How to Replace Basement Window Inserts