IIRC they are M6, try to find something socket cap as they tend to slip less.
Just got the callipers out and they are indeed m6. I’m struggling to find anything specific other than the m10 hope ones.
Pop by your LBS and see if they have some? I did have a whole box stashed away but it's in the lockup.
https://www.tartybikes.co.uk/other_hub_spares/tartybikes_axle_bolt_washers_pair/c596p14072.html?pos=1786.7166748046875&search_params=YTo4OntzOjg6ImluX3N0b2NrIjtpOjA7czoxMjoiY2F0ZWdvcnlfaWRzIjthOjE6e2k6MDtzOjM6IjU5NiI7fXM6MTY6ImNhdGVnb3J5X3JlY3Vyc2UiO2I6MTtzOjg6ImtleXdvcmRzIjtzOjA6IiI7czo5OiJicmFuZF9pZHMiO047czo0OiJtb2RlIjtzOjM6ImFsbCI7czo0OiJzb3J0IjtzOjk6InByaWNlX2FzYyI7czoxMDoiY291bnRyeV9pZCI7czoxOiIxIjt9
Browsing hub spares on trials shops might turn up something
@AlexD started
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IIRC they are M6, try to find something socket cap as they tend to slip less.