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Just picked up a used YT Tues frame thats missing the brake barrel mounts.
YT Canada are OOS, YT uk want 20quid in shipping to a UK address, doesn't seem to be any other ship out of country options for the other ciuntry sites.
Guessing the usual German online shops don't carry consumer direct brand parts so any other brands using a brake boss setup? I'm hoping it'll be a fairly generic sized part

its a barrel nut or cross dowel it looks like from the picture. Standard from fasteners suppliers. You need to check dimensions and thread but should be easily available as its just a generic thing - or looks like it
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Whyte bikes use them too, so maybe their website for spares will help?
A few seat clamps use that type of barrel nut, the older nukeproof one for example, there's lots of M6 titanium ones on eBay just measure the hole for the correct OD and whatever lengths they are, getting slightly longer ones might help installation of the adapter or caliper if they move about and fall out a lot.
Geometron also use them.