Since the part is not shown in the manual, there's a good chance it isn't supposed to be in the carburetor at all.
There is no functional purpose in putting two sealing washers one on top of the other, unless someone thought the main jet needed to be spaced down further, for example if the tip of the jet was fouling on the screwdriver slot of the emulsifier. Have you checked for graunch marks on the tip of the main jet and the slot of the emulsifier, due to them fouling each other before the extra washer was fitted?
Here is a rather unsupported theory for you. The previous tenant had carburetor problems with his Suzuki M120X (what a surprise). Somehow he decided that he could save the day by putting an extra washer on the main jet. However not only did it not work, it also caused the main jet to fall out in service.
