Created in conjunction with FirstSpear, Saber Solutions has introduced a Lower Back Pouch and Lower Back Panel.
The Horizontal Lower Back Pouch is designed for the Surefire 60 round magazine (MAG5-60).
The pouch is set in an ideal location for ease of access and frees valuable real-estate on your body armor carrier. Attaches to a plate carrier with a stiffened hook and loop panel and mates onto the flap that secures your plates or cummerbund.
Configurable for left or right handed extraction and features a reversible base for different plate carrier types.
Designed as an add-on panel for your body armor carrier and attaches to the hook and loop found on armor closure flaps and cummerbund flaps.
Provides a horizontal mounting platform for any items that you may need immediate access to such as grenades, firing devices, tourniquet, or magazines.
Stiffened to provide stability and rigidity. Reversible for different hook and loop configurations.
Apparently TYR tactical did it years ago.
Are you referring to the Magazine Rifle Pouch/Adapter – MV Low Back?
http://www.tyrtactical.com/products/details/magazine-rifle-pca/pico-mv-low-back-mag-pouch-556/