Received these parts from my German friends today – the ATC/Transponder knob is from  Cockpit Concept — looks completely identical to the genuine knob.

The circuit board is from Volker and amplifies the current from your existing I/O boards (such as those from FDS, or OC). Why do you need such a function? If you’re working with genuine parts you find there is insufficient current required to drive the annunciators (bulbs); interface boards on the market are usually only made for LED outputs. The board is an inexpensive solution, especially for the overhead switches and annunciators.

Take a guess, which knob is a replica?