it is. For that application, the key slot is quite odd, so I think the
I'm not an engineer, nor do I play one on the internet, but my guess is that the shallow slit has something to do with compressional force distribution. Since we cannot make the hole smaller, the 'grip' of the collar comes from deforming the shape of the hole around the shaft.
Without a slit on the far side, all the compressional force from tightening the screw is concentrated at the screw end. The round hole basically deforms most on that side, and the very least on the far side, making a teardrop shape.
But with the narrow slit, the 'teardrop shaped' deformation takes place at the base of the slit, in the center of the collar: not enough to close the slit down, but enough to distribute the deformation along the sides of the hole and effectively squeeze the collar evenly on both sides. Instead of a teardrop shape, it closes down like a clamshell with semicircular halves.
--humunculus