ALUMINUM FINISH ( MILL COLOR ) IS DISCONTINUED AND LIMITED TO EXISTING STOCK (2) >
The ALUMINUM frames moving forward will ALL be PAINTED WHITE 
C-310A-K Aluminum Basement Insert - Kewanee Replacement with Expander / Adaptor

Size: 31 x 20-3/16 + Expander = 20-5/8 

Columbia Series C310A-K Aluminum basement window replacement insert with insulated glass. 
Maintenance free and energy efficient. 
Easy, 1 minute Installation.
Needs no tools to install; uses high tensile steel spring clips to secure the insert in the opening.
Expander allows for use in most concrete basement walls with poured-in-place steel buck frames. 

Color: White Aluminum finish standard

Fits in buck with 7/8 to 1 inch drop-down (see third image)

Standard Sizes
31 x 12-3/16 + Expander = 12-5/8
31 x 16-3/16 + Expander = 16-5/8
31 x 20-3/16 + Expander = 20-5/8
31 x 36-3/16 + Expander = 36-5/8 egress window

Note: Basement inserts with expanders are used with Steel Frames, Aluminum Frames do not need the adapter. 

Priced and sold each.

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