I agree, I think it's a compromise for an aesthetic. I just put calipers on mine to confirm the 20mm measurement, but I wanted to mention that I used the nearest fit SAE size in my impact socket kit, 13/16", and it worked perfectly. It took a considerable amount of torque and it broke free with no spongy precursor to rounding off the ends of the flats, so if you can put your hands on a 13/16" you may want to cancel the special order 20mm.
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