It's not a wingnut. It's a strange M6x1.0 wingscrew, about 13mm long with a smooth narrowed tip on it. Strange, because at first glance it looks as though a phillips-head screw was merely screwed into a wingnut, with a flat washer permanently attached on the side that goes onto the lid. But it really is one piece (the three parts held together with an adhesive?). This thing got past the bean-counters as it seems to me that an ordinary screw with washer would have done the job. If the body of the screw is broken, you might be able to very carefully drill a small hole into the screwed-in part and use a very small screw-extractor. On second thought, you might try using a tiny magnet to attach to the screw and unscrew it - it's a fairly loose fit. On third thought, I didn't pull the cover off to see what access you have to that screw - you might try that first. Good luck!
EVs: Two '12 Wht/Blu SE Prem., '13 TeslaMS85, three 156v CorbinSparrowsLi(NMC), 24v EcoScoot(LiFePO4)
EV Conversions: 156v '86 Ram PU, 144v '65 Saab96, 48v1kW bike
RIP(2021) ICE: Orig.Owner '67 Saab96V4, '88 IsuzuTrooper; '76 MBZ L206D RHD RV