Steel of some kind.
I'm still not impressed. But I'll prolly keep them.
The sliding block wobbles as the screw is tightened. Ordinary 60 degree threads on the Leade screw. Fit and finish is poor. But they were not expensive.
The Bosche has Acme threads and it's smooth as silk with solid fit and finish. But Amazon doesn't seem to have them anymore.
So I'll prolly keep the KBC handles for now.