This is the breech collar with the transfer port in, so it's rather important..!
They are locktited onto the barrel. You need to take the barrel off, unscrew the 2 allen bolts on top of the action, the barrel should come out. (some newer S400s have a third screw underneath, to get at this, you need to separate the barrel and breech block from the cylinder. There are 6 allen blots holding it on. Don't lose the transfer port O-ring, which is in betwen the two sections.)
Carefully locktite the collar onto the barrel, making sure the holes line up. Make sure the loctite has dried, then replace the barrel.
Gus
The ox is slow, but the earth is patient.