u/Barcelus

I've recently inherited an old Kettler bike, and it has a very weird mounting system for seat. As in pictures, there is a smooth hole and part of seat pipe is visible through it. Definitely a part is missing, no idea where to get it.

Whoever owned it before must've lost it, hence the crossbolt to hold the seat pipe in place.

Any idea which part I'm missing and where can I get it? I live in Poland, Wrocław.