Don't know about the Grub Screw but that is a great looking rifle!

I've seen that same screw used on two Low Walls I owned, and a couple others I've seen for sale. Can't say I've seen it on a High Wall, but not sure why it wouldn't also work there?
The screw simply stops the sear engagement, and sets it shallower to give a lighter trigger feel. On mine I could turn the screw in until the sear wouldn't engage at all, and the gun couldn't be fired. Back it way out, or remove it, and the gun was "normal" trigger. Once it contacts the sear, it goes from minor adjustment to disengaging the sear completely with very little adjustment.
I liked the sear adjusting screw myself, and it worked well once set. I used some blue loctite on the screw to help it stay adjusted, but allow it to still move if I wanted to.

Reading through this post has been interesting.  If I was to add a screw for this reason to a gun I would use a very fine thread, something like #3-56 or even finer.  This should make it a bit easier to adjust than a more common courser thread.

I have a HW with the set screw.

Nero Hi,
            Looking at my action, the screw is through the bottom and at the front part of the trigger guard,I tried to load pics but no success,I sent some to n.r. Davis via email I could do the same for you.  The screw bears on the spring and changes the weight that way.

        Cheers Mal. Contact via PM.

Don't ask how I was able to post these.  A long time ago I found out that my "Puter" would not blow up so I just keep pushing buttons......

Many thanks for the photo's.
For anybody not computer savvy, If you just right click on the photo and then on the menu that appears click open in new window it will reduce the size.