The ALUMINUM frames moving forward will ALL be PAINTED WHITE 
C-310A aluminum basement insert - Monarch replacement with expander / adapter and dual pane glass.

Size: 31 x 12-3/16 + Expander = 12-5/8 

Columbia Series C310A Aluminum basement window replacement insert with insulated glass. 
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.
Expander allows for use in most concrete basement walls with poured-in-place steel buck frames. 

Color: White Aluminum finish standard

The Monarch was slightly smaller than Columbia and the Expander is added to the bottom of insert
The expander creates a 5/8\" thick leg that fits into the Steel Buck Frame.

Fits buck with 1/8" to 5/8" drop-down (see images above)

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