I saw that thread... if it wasn't Mister Yon, I'd say it was some kind of gag. But he lends a certain credibility to the question. I don't recognize it, but I'm not an expert on weapons-round-the-world, either.
There had better be an extendable tube that comes out the back of that thing, or that shoulder stock placement doesn't make any sense. Whoever fired it would be jumping in the nearest lake to put out the fire on their back. In fact, a shoulder stock doesn't make a great deal of sense anyway, given that over-the-shoulder launchers are generally recoiless anyway and often don't have any stock at all.
The presence of a shoulder stock implies that there is recoil and that fire does not shoot out the back. The scope implies some precision, and that the weapon is line-of-sight. I'd be tempted to think of some kind of grenade launcher than anything else. Not rockets... no blast shield in front.
It would be a distictly unpleasant way to lauch mortars.
So... Beats me. Mister Mag? Any help here?
Pretty much the same conclusion as I came too. That looks like a COTs scope which is odd. My guess was some kind of home brew but it looks too well painted for improv work.
The sheen makes me think it's all plastic. The scope is probably not more than 4x, and doesn't have a big objective lens.
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.