I agree. I spent a lot of time working in that area. I don't remember ever handling that piece. It could be that it fell out when I was removing the woodwork. It's one of those "FAIRYSNUCKiT" parts. The parts fairy snuck it into my parts box when I wasn't looking. I'll look if I can get in there to see. It reminds me of a piece screwed to the door frame to stop the rubber sleeved window counter balance spring from rattling, but it's not the right shape or size. Thanks mrfatboy and Alfred.
Tom Kizer