This is a PBS-1 Silencer.
It is a G&G model which you can get from pretty much any HK based company like WGC, Redwolf or UNcompany. There are also some chinese made ones now on the market from places like Gunners and RSOV. The G&G is nice because it screws apart for inspection to show it's not real (At least mine was when I received it from overseas)
The slim one linked above is the PBS-4
This is it mounted on a 104