Cut it just below the head and reweld it

Seriously, it's no different to modern seat posts. Of the two bits that clamp the saddle rails, the bottom one sits in a toothed curve allowing you to tilt the saddle. This is an old post so it's probable the bits have jammed together. Remove the bolt completely, the top piece of the saddle clamp will just fall off, the bottom piece should just fall off but it may need some gentle coercion.