It is designed for APS ACP601 Airsoft Gas Blow Back GBB Pistol.
It has 14mm + thread on one side connected to the outer barrel of ACP601 Pistol and 14mm - thread on the other side allow user to mount aftermarket airsoft silencer/flash hider.
It comes with a external cover so when the adapter is unused, you can cover the thread to enhance the beauty of your gun.