I don't know this particular engine either but I wonder... by chance has someone removed the thermostat but left the spring part of the stat in place? Normally on other motors the stat return spring is fixed within the stat but if it all came apart due to corrosion perhaps the previous owner pulled all the parts out but thought the spring should stay. Just a long shot??
EDIT: Just looked up a parts diagram and if it's this one it appears the thermostat capsule, spring, etc are all supplied as separate parts... not seen that before.