2x Sound Voltex MISUMI Resin Washers WSJW10-6-2
These washers are the Original Part used in Sound Voltex knobs.
Your knobs on a sound voltex machine should feel like they're gliding through, not scraping against any other part. The Misumi washer is what makes this happen, acting as a buffer between the volume knob and the top bushing!
If your current washers start to feel worn, or you can move your knobs up and down slightly, it's time to look into getting replacements.