Inspect the old piece very closely. Does it look cut? I can't imagine a thermostat failing like that with no repercussions. For one thing, the impeller is right there, the missing pieces would get caught up in there. If you can, look inside the water pump for any visual indications of coming into contact with the spring. If there are no scratches or gouges, then I'd say the missing piece was cut out. Is it a used car? I wonder if the PO did that in response to overheating.