What kind of freewheel/cassette is this? I bought some old bike from netherlands, I wanted to restore it.
Rear wheel is broken and I wanted to exchange it but there is this strange freewheel which I cannot identify, do you have any idea what it is?
It's called a Helicomatic. The idea was to make a freewheel that does not need a lockring that has to be torqued tight. The helical splines make the freewheel tighten itself.

Amazingly we had this exact question about a month ago, so I flagged this as a duplicate question.