what is that little PCB on the metal shield for? I din't have that in my setup
Is that extra PCB perhaps a video inverter to invert the RGB signals? I would be interesting to know which make and model your monitor is.
Yes, that's an inverter board for use with standard games.
His model looks like a typical 20EZ. Some came with it, some didn't.
But you also need the single red power wire in order to use it for standard games.
In a Nintendo game like yours/his it's actually just bypassing that board and more or less going straight to the chassis in the same fashion as yours is without the board.
Non-Invert Out = Nintendo / Invert Out = Standard game
Long story short - Donkey Kong does not use the inverter board.