This is a much better than average quality aluminum crush washer, nearly 1.5mm thick for positive sealing.
10mm internal diameter aluminum sealing washer with small outer diameter, commonly used for brake hose banjo bolt applications, on Guzzi models from the mid 1970s through the early 2000s. Alternative to the copper version 25656100.
Also used for cylinder head oil delivery line nipple on 2 valve per head square fin 1000 and 1100cc motors, from 1984 through 2009.
Also for fork bottom bolt on early disk brake Tonti frame models and others with original Moto Guzzi forks. Some Guzzi parts references list part number 10528900 for this particular application, this narrow shouldered 25656102 crush washer is correct for that use.