The black block cube looks similar to the tiller attachment on my Danish boat
If it is the same principle as mine,you have to remove the 8 mm thru bolt,as the bolt passes thru a notch in the rudder stock.
The little screw on the top is used to set the height of the tiller arm for comfortable steering
The 8mm thru bolt is what you attach your tiller to & it allows my tiller to be stood up vertical to get it out of the way when wheel steering
My personal experience & humble opinions-feel free to ignore both
Remember-If all else fails, Google is your friend.