I pulled my dash and all of the HVAC components a while back to clean out all the leaves and replace the disintegrating foam on all of the various flaps and doors. I took a bunch of pictures during disassembly, and now that I'm reassembling everything there's a part in the pictures that I seem to have misplaced.

This circular washer or spacer. It's hard to tell what material it's made of and how thick it is from these pictures.