I mounted this on my shotgun which I sold before firing a single shot.  Item is listed as used but condition is as good as when I purchased new.  Still have original package, instructions, and allen key.  This mount sells for $38.99 + shipping on CDM gear website (what I paid).  I am selling for about 1/2 price... and item is as good as new!

Precision machined, high quality mount has built in anti-rotation QD attachment.   You can also add an optional rail piece (not included) on the opposite side of QD socket for mounting a light or laser to your shotgun.   Please see description for more information and fitment.  See CDM gear website for options available.

Fits 12 ga. shotguns with 1" of space on a 1" magazine tube. Fits Remington 870, 1100, 1187, Benelli M1/M2/M4/M104 (not SuperNova), Mossberg 500 7 shot, 590 (not the 590A1 or SPX models, see MTC clamp), Winchester 1300, 1200 and SXP Defender. 

** THE PICTURE ABOVE SHOWS THE MOUNT ATTACHED TO THE SHOTGUN.  THIS IS FOR ILLUSTRATION PURPOSES ONLY.  THE SHOTGUN IS NOT FOR SALE!

Info from manufacturer's website:

The BMT magazine clamp  for 12 ga shotguns, weighs only 1.5oz. and is lighter than the factory Remington stamped steel clamp that comes on the Remington 870 with an extended magazine tube. You can mount a variety of rails and sling mount systems on this clamp. It can use optional 5 slot or 3 slot rails to put the light and clamp exactly where you need it. One tool for one bolt is all that is needed to attach and remove this clamp in seconds. Comes with all wrenches and 3M polyurethane no slip grip pads to protect your finish. Includes a built in non rotational sling swivel cup for QD PB sling swivels. With illumination and targeting systems getting smaller, lighter and more compact, we're moving with the industry, law enforcement and the military to provide a superior mounting platform that provides for a stronger, less complicated attachment for your weapon.