Originally Posted by bulgie

That's called a "head clip". Associated mostly with pre-war English bikes, mostly gone by 1950 though there were a few holdouts, like the Cinelli track sprint bike as late as the 1973 catalog still came with a headclip.



The steerer has a slit in it, and the pinchbolt clamps the steerer down on the stem. The English stems for headlips have no cone or wedge or expaner bolt, they're only held by the headclip.
The Cinelli sprint bike used a regular Cinelli steel stem with a cone, but the headclip added more stiffness. Regular stems, that tighten at the bottom only, always have a little slop between the quill and the steerer up at the top, so that sprint bike used 'belt-and-suspenders' for the ultimate solid connection.

Oh they also used it on their stayers (motor-paced track bikes), here's a Cinelli stayer, '50s I think: