Anyone buy a PC 7336 random orbit sander recently?

I bought one yesterday and the bag that contained the wrench and the manual also contained a thick plastic washer about the same diameter as a dime. Does anyone know what it is for? The manual doesn't mention it and I can't find it on the parts diagram.
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Any numbers on it? I assume you looked at the online parts list. Maybe someone else can find it. http://media.ptg-online.com/media/pc/Parts%20Lists/20021115200000_7334.pdf
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There aren't any identifying marks on the washer.

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I'd guess that it is a backup replacement for a commonly worn part. Maybe between the orbital disk and the shaft turning it or something like that. I'll assume it has a hole in the centre of it?
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