Just get a 19mm seatbinder bolt (they're very common and very cheap) with a notch and carefuly file the notch away.
Or get an overpriced Retro/Campagnolo seatbinder bolt.
@Carabo started
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Just get a 19mm seatbinder bolt (they're very common and very cheap) with a notch and carefuly file the notch away.
Or get an overpriced Retro/Campagnolo seatbinder bolt.