Instead of being "illegal" or a SBR at one time could it just have a pistol length upper installed?
In the photo it looks to have a barrel shroud thingy but we don't know it's length and it very well could be a full 16" barrel in there.
I personally think it's a really cool piece and I'd snap it up if it were available to me. I've been wanting to start collecting old open bolt semis for a while now.