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Sorry, this is @adamdenw
You can buy the convertor bolt (and anything else you need from Spa Cycles to make the most of the postage cost).
https://spacycles.co.uk/m7b17s100p2341/SPA-CYCLES-Allen-Key-to-Nut-fit-Converter-Bolt
If it needs a nut-fitting brake front and rear then the rear allen-sleeve-bolt brake with the really short bolt gets converted and goes on the front and the old front brake goes on the rear.
No, traditionally you couldn't use that brake and mount combination but now there are some adapters but you will need to +20mm
There isn't physically enough space between the posts on a post mount to fit a flat mount caliper and adapter without adding the extra 20