Necchi Novas and Miras are unusual, in my experience, in that the handwheels don't ride the shaft, rather there is a piece that fits over the shaft.
On some it is a large circle, as in the above photo; in others, it is smaller. It may or not have the brass washer. (These early Necchis were made one at a time, and vary considerably... parts are not necessarily interchangeable from one machine to another... a challenge and a frustration at times!)
In either event, that piece locks onto the actual shaft with two set-screws. One has a flat end, and the other is pointed. There will often be a tiny round piece of soft metal (aluminum?) in the hole the flat screw goes in, that fits between the set-screw and the handwheel shaft. You'll want to catch this when it falls out and replace it when you're reassembling the pieces.
Often, the point on the pointy set-screw is worn down... if it is very worn down, it may not be functioning and the handwheel may act like the clutch has been disengaged for some mysterious reason. This set-screw is the reason.
The pointy set-screw needs to lock firmly in the groove in the actual shaft:
I have run into machines where this set-screw would not stay put (wear over the years) and needed a drop of Lock-Tite (the kind that is not permanent) to keep it in place.