C-400A Vinyl Basement Window Insert Replacement - Monarch Replacement with Expander / Adaptor and Dual Pane Glass. 

For standard 31 x 12 inch basement windows

The Columbia Series 400A vinyl basement window is an 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. 

This basement window insert replacement is available in white vinyl only.

The Monarch was slightly smaller than Columbia, and the expander is added to the bottom of basement window insert replacement
The expander creates a 5/8 inch thick leg that fits into the steel buck frame.

Fits in buck with 1/2 - 5/8 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


Replacement basement window insert priced and sold each.

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