This is a ProMag known as the Magazine Clamps, model PM016B.
The ProMag magazine clamps hold two AR-15/Mini-14 steel or aluminum magazines side-by-side for quick reloading. The clamps position the magazines and the loaded ammunition pointing in the same direction. This four-pack will "jungle clip" two pairs of magazines for ready use.
Fits AR-15/Mini-14 Magazines
Includes 4 Clamps
For use with Steel or Aluminum Magazines