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I meant 650c, like lo-pro front wheels and old TT bike size. The wheel diameter is very very similar to 26" so should be well within tolerance for nearly any standard rim brakes. It's an unfashionable size so you can get deep Zipp wheels for £20
But that's a super nice bike man, did you have to do anything to get the brakes working or was there enough adjustment by default? Are they the Brevet rims?
Edit: ah you answered my questions in your edit :)
Did 650b on my gravel bike, 650c would be fine and you would probably have a couple more brake options (these are maxed out tektro CL720s.)