Those bolts sometimes have a key on one side that fits into a keyway in the frame to stop it turning, so it's not surprising one side is solid. The bolt looks pretty far gone, I'd just cut it.
Keyed bolt: https://www.sjscycles.co.uk/seat-posts/vintage-style-steel-seat-post-binder-bolt-16mm/
@ffm started
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Those bolts sometimes have a key on one side that fits into a keyway in the frame to stop it turning, so it's not surprising one side is solid. The bolt looks pretty far gone, I'd just cut it.
Keyed bolt: https://www.sjscycles.co.uk/seat-posts/vintage-style-steel-seat-post-binder-bolt-16mm/