From Magpul:

CTR Carbine Stock Mil Spec

The patent-pending Compact/Type Restricted (CTR) is a drop-in replacement for the standard M4 stock body. Designed for stability, the CTR utilizes a shielded operation lever and a friction locking system that secures the stock to the buffer tube for zero movement.

Note: There are two types of buffer tubes (receiver extensions) in common use: Mil-Spec and Commercial. If you have a Commercial buffer tube on your rifle, make sure you order the Commercial CTR, stock number MAG311. 

LOP Adjust= 3.3 inch

MIL-SPEC CTR RESOURCES

Features

• Friction locking system that eliminates all stock movement.
• Sloping cheek weld for improved user interface
• Ambidextrous sling mounts for QD, 1.25" loops, and lanyard hole
• Side mounting slots for cheek risers

Specifications

• Weight: .0785 pounds without rubber butt-pad or spacer butt-pad
• Collapsed Length: 7.2" without rubber butt-pad or spacer butt-pad
• Extended Length: 10.5" without rubber butt-pad or spacer butt-pad

How to tell if you need Mil Spec or Commercial version stock? Click here!