Nothing.
The sight/scope rail is not connected to the shroud in any way. As you can see by looking at yours, there are no mount, screw, clamp locations. The shroud just slides past the rail base.
As for the block (real name ?) that's on the end or your shroud...these two are screwed together. Just unscrew them. Mine did not have any epoxy or sealant on it. it just unscrewed. Two pieces of clean, soft rubber sheet may help you with the grip on the two parts, grip tightly and untwist it (standard threads).
Mike