The brass is the piece. It starts as a flat strip thicker than the sheet used elsewhere in the range, annealed until it will move, then hammered round the stem a little at a time so that it draws in evenly instead of buckling on one side.
Because the collar is heavy, the wood underneath has to be cut to receive it. A shoulder is turned into the stem for the brass to sit down against, and the seat is fitted to that particular strip of brass rather than to a standard measurement, which is why collars are not interchangeable between pieces.
Once seated, the collar is filed flush at both edges and burnished. The hammer marks are left in rather than polished out, so each one carries the record of how it was closed. The stem is kept short so the weight of brass stays balanced in the hand.