Posted by roy entwistle on 22/03/2018 12:36:22:
The last photo of the chuck shows under 36B, Thread 5/8 x16 So presumably it is screwed onto the spindle
Roy
Don't spoil things by pointing out the obvious!
Quite possibly the previous owner took the spindle out and machined down and threaded the jacobs taper to suit a chuck already in hand.
One possible route is de-rusting, dismantling and reoiling the chuck which could be done with the spindle removed but the chuck still in place.