Turn the bolt back a bit so that flat is parallel to the flat on the stem. Put an Allen wrench in between the 2 flats and it should hold to undo the nut.
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72 Geoffery Butler, 72 Gugificatizion Witcomb, 72 Raleigh Gran Sport SS, 74 Raleigh GrandPrix dingle speed, 74 Raleigh international, 81 Centurion Pro-Tour, 85 Gazelle Primeur, 29rBMX, Surley Steamroller 650b