The shifters, and the whole bike, has veritably no use to it but has suffered from decades of storage neglect and superficial damage. It's an interesting story, a true barn find, but I digress. So, while there is no damage from use, the little black washer has deteriorated (see pic - this came from under/behind the screw head). I believe that this is the part that is causing the shifter to fail. If so, does anyone know how to replace this? Is this a rubber washer or maybe a fibrous material? I assume that this washer is designed to add friction. It is this friction component that I need help on. Does anyone have experience coming up with a solution to this problem? My tact is to keep these shifters, and all of the components original to the bike if possible.

Thanks for any help.