> The 258 seems to be described as a vco, and the 259 as a
> 'waveshaper'
The 258 is a "Dual Oscillator" and the 259 is a "Programmable Complex
Waveform Generator" is also dual but have a dedicated modulation VCO to
the "main" oscillator.
you can compare schematics for the 258 and 259 here: