A few more links,

- This shows the two 150 variants, mine with the wedge/pin is the same as that in the lower two photographs, butting up to the straight shroud, with a small portion of just barrel protruding at the front.
The top two pics show the screw/clamp arrangement.

-Waffencentre have this type for the FWB300 (in this case one of three variants), not identical but similar.

If you end up considering fitting the 300 model I can measure mine up and/or compare it with a 300 model from another club shooter to see if they are interchangeable.

Good luck in your quest