To be completely honest, I wish I had an answer for both of you...but, I don't. I wanted to wait until I got home to reply, as I wanted to test the piece that comes with the SX107, as well as the pieces that come with the AXIC3009 kit. Having tested them, all I can say right now is, I'm thoroughly confused. Let me explain.....
First, I tested the bottom pieces from the AXIC3009 kit on a standard servo having a 25T spline. This was how I determined which piece was for a 25T servo spline. Next, I tested that same piece on the MG59...and, it's too big. Looking at the two servos, I can easily see that the spline on the MG59 is smaller...so, thinking it might be the same as the spline on the SX107, I removed the complete servo saver from a dead SX107, and gave it a try...and, it's too small.
Obviously, something is NOT right. As for why I'm 'confused', I pulled out my social calipers, and measured the diameter of a standard servo's 25T spline, and it measured 6.0mm. Next, I measured the SX107's spline, and it's 4.85mm. Finally, I measured the spline of the MG59...and it's 4.95mm. Literally, 0.1mm larger in diameter than the SX107 spline.
I'm going to call PowerHobby tomorrow, to see what they say. Honestly, I hope they have an answer. Maybe they're not even aware, and are under the impression that the spline is exactly the same diameter as the SX107 spline. Maybe the factory "goofed" in manufacturing the spline parts. Whatever it is, I hope to be able to reply back tomorrow with an answer.
In the meantime, does anyone know the tooth count on the SX107? If it's 25T, then there's the possibility that the bottom pieces of the servo saver could be pushed down with a little extra 'forced'...although, I'd prefer not to do that...which is why I'll be calling them tomorrow.
~ More peace, love, and kindness would make the world a much better place