Seen. Now if I wern't so bloody tired, and actually allowed my brain to process what my eyes saw, the correct answer would have been "It is fastened in place just ahead of the barrel nut. The large block at the back end of the hand guard, has the top portion separate, the bottom as an integral part of the receiver portion of the chassis, they are held together by 6 allen head bolts that clamp the tube in place"
edit: If I remember correctly, the tube is indexed to reassemble in a repeatable manner.
Hope that helps. And my bad for the miss read.
No worries, thanks for the info and description.

Mark