I borrowed a seventies Puch Super Sprint from a friend after my commuter died. It has down tube friction shifters but the rear mech kept gradually going up through the gears by itself, particularly over bumps and things so I concluded the shifter needed tightening up. I'm not very familiar with road bikes but the shifters on it are tightened via a screw with a hexagonal head and around the head they have a plastic bracket (pictured) that looks kind of like a wingnut and allows you to tighten the shifters with your fingers. The problem is they're both cracked, which I wasn't aware of, and the moment I tried to tighten one of them (whilst cycling) it just crumbled off.
The guy who owns the bike plans to sell it as soon as he gets it back so obviously I'd like to try and replace the broken part but I can't figure out what this thing is or where I could get another. I took it to my LBS and they didn't have a clue but they're never been very good with old bikes.
Any suggestions? I just need to know what to google for.