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I think it would work, I have those brakes and those guards on different bikes, the guards were squeezed a bit too make them 650b from 26" but they're about 52mm on the outer edges. The brakes appear to have about 65mm clearance at the level of the guard, plenty of room for the 45mm chromo-plastics I have on that one. I'd have taken a photo but couldn't get anything useful in my dark shed.
Polygon-
Thanks to you and @kvragu, for your replies! After going to the shed to look at the bike, I think I might stick to the 40mm width. It’s going to be my winter roadie, and I can’t see myself going bigger than a 32 for that purpose (the guards are rated for 32 max).
If I want to stick anything larger on it, it’ll probably be for terrain where tight clearance guards wouldn’t make sense!
Arnomatic
Looking to get a set of Gilles Berthoud guards for my winter rim brake bike, does anyone know if the 50mm width would fit under my Tektro R559 callipers? They have a lot of clearance, but without the guards at hand it's hard to picture. The 40mm guards clear a 32mm which is probably enough, but the bike can fit larger so it would be nice to have options. Cheers!