I don't THINK (rpt, "guesstimate") that the ring is missing. I'd normally only expect to see a 2nd (upper) ring on a keyless chuck, and yours is the key type.
The chances are (and I don't know this particular machine), when you turn the arms to move the chuck down to it's lowest point you will reveal a rectangular slot in the inner quill (you may have to rotate - by hand - the chuck about half a turn for that slot to show up).
If so you should have got with the drill (or can easily make) an "angled drift". That's a piece of steel plate about quarter inch thick, about 6 inches long, and tapered on one side so that in "flat view" it's about 2 inches wide at the thick end and perhaps something less than a half inch wide at the thin end.
To operate, having exposed the above slot, insert the drift, small end first and angled side downwards into the slot. Then just tap gently with a light hammer. This will break the (friction) lock between the internal taper bore of the quill and the external taper of the arbor to which your chuck is attached.
CAUTION: BEFORE starting, arrange a thick pad of cloth and/or piece of softwood under the chuck onto the table because the tap will release suddenly - you do NOT want to damage either the chuck or the the table by having girt big lumps of metal (your chuck) suddenly arriving on them at a million miles an hour. Similarly, before starting the above, wind the chuck jaws fully open so that the tips of the jaws are "hidden" into the chuck body.
The above is, IMO, the most common method of retaining a chuck onto a pillar drill. But according to manufacturer there are several other methods. These could involve 2 special "wedges", undoing a special central machine screw (inside the chuck), etc, etc.
If no one comes along soon who knows your specific machine I suggest you look for the above slot in the inner quill (as described above). If you can't find it then I suggest you take a couple of pix (1 chuck fully up, 1 chuck fully down) and publish them here. Then we can have another go.
Removing a chuck is normally not very difficult - it's "just" a matter of finding out which "trick method" your particular manufacturer has chosen.
HTH
