What is it ?

What is it ?Attachment 106682

here is a clue for the part posted Attachment 106689

Mounting rail for the M203 grenade launcher...and top handguard...

not a mounting rail but related to the M203

Well, it slides on it...here... Never really knew the name for it.

You are correct, it is the barrel extension part # 8448315, when you slide the barrel forward it cocks the trigger