I've successfully replaced the pinch roller, belts, and rubber rollers in my BeoCord 2000 with DKSoundParts' excellent kits.  After reassembly I noticed this clear plastic disc or washer in my work area, and can't figure out where it might have come from.  I've looked through the service manual and can't find an obvious candidate either. Always possible it's not supposed to be there -- this deck had been repaired once before in the 1990s and it might have dropped in there while on that workbench... I've photographed it with a pen for scale... thanks for any ideas!