I believe the colouring of the chassis/panels is slightly different than standard, and by far the largest difference comparatively is the top rail being specifically for the Stiller MK13 action -- it uses a unique hole spacing with dowel pins as additional recoil lugs. While there is a physical separation, the top rail is inline/considered continuous with the top rail of the fore-end as well.
Others with more direct experience will jump in I'm sure, but the above are the two largest differences.
