It looks like a QD bushing / coupler that just might be stuck on the shaft of the ball screw. I can't really see 100%. Judging by the rust the shaft is just rusted to the coupler. It looks like it may need heated up a bit to expand the female side of the coupler. But just be very careful you aren't going to burn anything.
Take a look at the catalogs and you can see how they work.
GGMFG > Products > Couplers/Clutches > Coupler Hubs
https://www.mdsofmi.com/product-cate...s/bushings/qd/