This is a M12x18 crush washer.
Commonly used for the clutch slave cylinder fluid connection on Lancia Scorpion and Fiat X1/9 cars. One required for Scorpion, two required for X1/9.
Also used for fuel inlet banjo fittings on Weber DCNF carburetors and on oil pressure sending units for Fiat 85 cars. Copper or Aluminum construction.
New and unused.