Yes ! You're right, I just read too quickly ! sorry

So i seached a while and found no spare fader cap for tr-707 unfortunately...
Parts numbers 2247037100 (for white) and 224703700 (the orange one) are of course useless.

By googling "Roland Fader Cap" I found interesting replacement that could fit, refering to alibaba sites

I also found this  on ebay after googling "roland knob cap" in google Images !
SH-101 MC-202 x5 Orange Slider Plastic Caps knob sh 101 Roland Cap knob Part

 

It's for sh101/mc202. Are these cap "compatible" as they look larger...
I do not own these gears, so if somebody could tell that some parts from some synth can fit on others ones

Personnaly, If I find better cap, I would  replace mine...
Originals fader cap sound ugly to me :-)