Let it soak in penetrating oil for a while. Maybe a week. Bang on it with a hammer firmly a few times every now and again. Soak again. Try to remove it with a long extension. And repeat.
Or just buy an NOS one for $20. I am stubborn, so I would go the above route.
jim
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Cross Check Nexus7, IRO Mark V, Trek 620 Nexus7, Karate Monkey half fat, IRO Model 19 fixed, Amp Research B3, Surly 1x1 half fat fixed, and more...
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SB forever