Hi,I just ran across this on a recent vise disassembly. There was a retaining ring (roughly the thickness of the two combined in your photo) with a small gap in the ring around a recess in the shaft. I heated the ring red hot with a propane torch, stuck a pair of stout needle nose in the 1/8" gap and pried the ring opening wide enough to slide off the shaft. My game plan is to reverse the process at reassembly time.
The diameter does look slightly larger in that area - do you have calipers and can you get in there to measure? I suppose you could try pressing the spindle out of the casting, see if it moves.I'm starting to think the shaft was made, indents were added which pushed up metal around and then the collar was press fitted.