It’s what we call a “freewheel” but you need the old type (spiebaan in plaats van splines).
You don’t see them very often. The nut is also difficult, it’s M12x1.25 left thread. I do have those (I -sort of- own a Boxer) but I don’t have a freewheel for you…
I expect the number of teeth on the gear differs between a Ciao and Boxer, so I will check that for you (actually my friend Ritze will have to check this 🙂 ).
So… I asked Ritze about the freewheel on our Boxer and he said it has “20 or 21 teeth”.
Ciao has 18 teeth, due to the smaller wheels. You could use one of those, of course.
By the way, is your freewheel broken, missing or…?
Now about the weird thread on the axle: there is a M12x1.25 left hand version and a right hand version. You have left hand. I thought I did, too. But you’ve guessed it… Mine has right hand thread.
I do want to have some left hand nuts in stock, so I will see if I can order some. I got a quote about two years ago and I remember they were quite expensive (you have to take at least 25 pieces).