The nut has jumped ship.
It's an M6x0.75 nut, 2mm thick.
I've done a bit of searching online:
http://scemosystems.fi/en/m6x0-75-rh-9-2mm-zinc-plated-steel-jam-nutOnly one I found....
It seems that thread size is also used on some SMC products, so an Automation Supply House may have it.
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The pic above, with the switch riveted to a bracket, that's for the large-pivot/coffin Brembo masters.
Which is not what you have.