This product available at JK Army really has a long name, the C&C Tac Steel Short Bolt Set with M1913 Rail Folding Stock Adapter For Marui TM M4 MWS GBBR. This item has funtional Semi/Auto Sear with a Short Stroke BCG for better gas efficiency. This comes with  an  M1913 Picatinny Rail Stock Adapter For MCX / MPX - Style Folding Stock.

Set Includes:

• CNC 45# Grade Steel Bolt Carrier x1
• CNC 45# Grade Steel Recoil Spring Guide Rod Set x1
• CNC T6 Aluminum Adjustable Half Cock Sear x1
• CNC T6 Aluminum M1913 Picatinny Rail Stock Adapter x1

Compatibility:

• For Tokyo Marui TM M4 MWS GBB Rifle Airsoft Only"