They're notched on both sides and they look center drilled.First pic looks like it might be extra camlock studs or stops of some sort. Are they notched on one side?
Second appears to be an adapter sleeve for using a dead center in the headstock taper, maybe an MT5 to MT3?
Third I have no clue.
Fourth maybe a spider on top and a backplate, without camlock studs, on the bottom?
The sleeve is a self holding taper on it. Clean the surface up on it as well as the taper in the spindle and carefully slide into headstock spindle, twist slightly to seat it against the taper and it there! Tap it in place with a soft face mallet or brass drift. Don't beat the crap out of it! To remove, use a long drift thru the back end of the spindle, catch the back of the sleeve and drive it out......I was thinking the same about the sleeve but how would you secure it in the headstock?.........
Probably not. Let see what others say.Maybe to hold the Follower Rest?