Wow. Interesting. Lockbar sights too.
The person that told you it's stock from the factory is either lying, or doesn't know what he is talking about. I'd be skeptical about that rifle. Yeah, it looks like a standard FH cut down, the barrel does look a little shorter than 18.5 but I think it looks longer than 16, like the Socom's. And yes, that's not a factory stock. Unless at some point they were shipping USGI stocks with their guns.
However, it's possible any mods were done by someone that knew what they were doing. I have no clue. One thing is for sure, it's unique.
Plus, after looking at it again, I don't know if my eyes are playing tricks on me, but it almost looks like someone actually did that cutout on a factory stock to possibly have the USGI look. Not sure if I'm crazy there, but.....