This is a BLACKHAWK Holster known as the Magazine, model 40AM03BK.
This versatile" fully ambidextrous holster gives you the option of belt-mounted or inside-the-pants configurations. Fits belts up to 1.75" wide in a traditional vertical position and fits belts up to 1.5" wide in horizontal position (middle of the back). Includes a spare magazine pouch with adjustable retention flap.
Features:
Removable spring-steel clip
Reversible, non-stretch hook & loop retention strap with molded thumb break
Size 3
Fits 4.5-5 barrel large autos