So you are missing the nut to draw in a 5C collet or a chuck mounting arbor.
Are there any cap head screws visible on the face of the chuck? If there are then these are most likely holding it to the index ring so undo those.
If there are no visible screws then undo the clamp screw at the rear of the indexer and then screw off the clamp and slide the spindle out. You can now see if there are any screws countersunk or counterbored through the index plate holding on the chuck