Kitchen wall fixing component

We've been given a load of old kitchen wall cupboards which we're trying to hang in our utility room.

Unfortunately one of them is missing a small part of the wall bracket mechanism. It's a rectangular threaded washer, 10 mm X 5mm. When threaded on the bolt it's part of the mechanism which tightens the cabinet against the wall and it also stops the metal wall hanger from pulling out.

Does anyone know where I can buy these?

View attachment 70672

Thanks,

Chris.