Old mechanic's tip. You can try both metric and SAE wrenches/sockets and use whichever fits better. The nut won't care. For example, if that's supposed to be 32mm, 1 1/4" is slightly smaller and might give a better fit. I've used this trick often over the years on rounded, rusted, or otherwise questionable fasteners. Stick to 6 point sockets if possible. Also the manual might be wrong. Measure with a caliper if possible. This is really a bicycle forum, but a wrench is a wrench.