Side rail? Interesting. I wouldn't screw someing like that into my Scrolls but I can see the sharp gear edges can get painful. Practice Beethoven 6th's storm movement on the Extension, the individual 16ths and you will see with each bass what needs to be done. You can even buff the edges of the gears to get the sharp edges softened. One old bass gears, many of them are worn and smooth by now. My Tarr is very smooth now.
Speaking of
the Tarr, the page isn't up yet but I have some pics. They are dark and I will probably re-take them or lighten them up for the page but here's a look at the Extension Arnold made for it;
The fit of the Extension and String on and around the Scroll was a challenge on this being that the face of the head is so thin. Arnold made it work both artistically and structurally as well.