G&P knockoff of Mesa Tactical 870-AR15 stock adaptor....
 

http://www.wgcshop.com/pcart/shopper.php?itm=GP-ACC-391

http://www.gp-web.jp/photo/gp391_n.jpg

*fap*

And it has a built in gas reservoir, so it can be used with the Maruzen CA870 OR M870......groovy.....

I wonder if this reservoir could handle propane

ah shit.... there goes my next paycheck. If wonder if its like the PM Concept TSEE kit, which uses a CO2 shaped charged gas tank which does in the pistol grip.

EDIT: Erp well it appears theres "a large gas tank in the stock tube" apparnetly!
Also found additional information and pictures here: http://www.gp-web.com/en/productspop.php?pid=923

.....*fapfapfap*