Don't have that machine, but a knob in that position is typical for disengaging the line spacer cog so that you can turn the platen freely to align existing text with the machine's printing line - as when you put a sheet back in the typewriter to continue working on it.

But one does wonder why the thing is knurled if the only function is to be pushed in. Aesthetics or two different controls in one knob?
 

Rudi wrote:

Well, I found out that when I push the button, I can turn it about 15° counter clockwise. The knob then stays there about 5mm deeper in the machine as before -- and does nothing. It may be a mechanism called "Stechwalze", which should decouple the platen. Since the line feed mechanism is also on the right side, it could be related to such a functionality. But I can't figure out how this would work.

Aha! That would explain why the push button is knurled - you can lock it in place. Those with vaster experience can comment on whether they have seen this variant before. As to how it would work, does the platen stop clicking when you turn it when the button is in? That's about all there is to it. The button usually disengages the line stops or whatever you call them so they hold a new setting, whereas there is often a lever which has a similar function but forces the stops to line up with the previous settings when released.
 

I was idly fingering the left end of the carriage on a post-war Rheinmetall KsT when I noticed that the button you push in to disengage the platen was also knurled. So I pushed it in a turned it and sure enough it locked. It's unlikely I ever would have discovered this feature had it not been for this thread.