My SWAG is that this is a semi-disposable ripoff of the German Armbrust which is a short-range, silent (until the warhead goes off) anti-tank weapon. It uses the same captive-piston design used in silenced M79 and 12 ga ammunition developed during Vietnam. In this case, the main component of the weapon is a heavy-walled steel tube. The propellant is sandwhiched between two heavy steel caps, which push the projectile out the front, and a countermass out the back, making it recoilless. A heavy steel ring on each end stops the caps, and traps the hot gas inside.
The countermass is a bundle of soft plastic strips that weigh the same as the projectile. There is no noise (actually, more like a pistol shot), no smoke or flame, and you only need a couple of feet of safe area to your back. Great for ambushes from buildings.
US Special Forces have a limited supply of these specialized weapons.
That's my SWAG. The modest rifle-type scope, for me, rules out both an RPG or manpad weapon, since it would be inappropriate for either. The seeming plastic construction makes it just a holder for something else. I don't think there are missing pieces to make the thing longer.
A peek at what the ammunition is would answer all the questions.