I can't remember where I got this drive centre from, if it came with the sorby Jaws.
If I do mount like this, quite a bit of the screw length is lost.
Does it just go in the chuck without the Jaws, or some other method?
I think you're suggesting this?If you look at your first photo the bottom of the screw (on the bed bars) locates in the small arc on the inner part of the jaws on mine & holds very positively.
Not sure whereyou can use a wooden disc as a spacer.
Sorry, you posted this whilst I was posting mine...Hope that helps.
It looks identical to my Record power screw chuck /SC3 jaws. The instructions say (!) leave the jaws on but ensure the flat part of the screw chuck shaft is gripped by the sliding (pink) bits. So yes the protrusion is reduced. The regular jaws locate in the groove on the screw chuck, which would prevent it coming off if the pink jaws slip (they shouldn't). The work seats against the regular jaws. So once you've turned the spigot/groove, you can reverse it into the jaws which are already in place. (remove screw chuck....!)Here's a sketch of the chuck with no jaws in, the pink parts are the sliding thingys...
I've not used one for years but you definitely use it with the jaws attached to the carriers - the pink bits in your drawing.ok, so it sounds like the answer is that some put it in the chuck without jaws, and some with.