I’m not looking for caps made by another manufacturer that are different but fit, Mike - I’m looking for official black caps / fader from Elektron, if they make them. Thanks.
If they were identical it wouldn’t be too bad, but their caps look really different.
Necrothreading for the posterity 
Chemipro Oxi brewing cleaner removes the yellow stuff from the rubberized knobs (mix the knobs in a glass of hot water and a spoonful of granules for a few minutes).
This stuff?
Yup!
Where did you find those black ones? Looks ace
bought the knobs directly from elektron.