Beware of the design itself.
The way Castello executed it, anyway.
The shank extension is made from the same material as the stem, and the two can actually fuse together under the right conditions.
The catch is no one knows exactly what those conditions are. Some combination of surface contact area, pressure, heat, etc., of course, but not knowing precisely means it's a matter of luck that not much can be done to avoid. One day you give the stem a twist and it she no move. Try to force it, and the extension shears from the shank or the stem snaps.
I've lost count of the number of cases I've seen. One sent to me by one of the most well known pipemen in the world, so it's not a lack of experience thing, either.