The odds are 99.999% that it is a cut down 20-inch Carbine.  That stated, if you send me clear close-up pictures of the front sight ramp. the barrel band, and the muzzle face, I can most likely confirm if it has been cut down, or not.

Bert – [email protected]

[email protected] said
That would be a great find if it was the real thing.  

Unfortunately, it is not “the real thing”.  The front ramp sight re-attachment is not clean, the muzzle face in the white, and the front barrel band installed backwards.

Bert