Guitar Hohner ST59

Dear forum members,
I own a Hohner ST59. A few years ago I had this guitar tuned. But I am not sure if people were aware of the special black box present in this type of guitar. I received a very briefly drawn diagram of the electrical wiring through the company Hohner, but they had no information about the blackbox themselves. Do you have a better wiring diagram? I would also like to know what type of potentiometers were used in the original guitar (linear or logarithmic) because I think that this is important in the case of the special way in which these 3 potentiometers have an influence on this blackbox. I would like to replace the potentiometers with new ones. Do you have an idea which brand is most suitable. The resistors are 250 kOhm.
With kind regards,
Luc Martens (Belgium)