Thanks all for your input - I'll add the PCB schematic here for interest. The 'Target Hit' circuitry. (It includes 'UFO Hit' for a comparison).
Initial glance looks like an oscillator with a one-shot. From my knowledge of circuitry (and from listening to the difference between the sounds in question), I would GUESS that other Invader boards would have this same circuitry but different component values, as the invader 'hit' sound appears to be based on a faster oscillation than what is heard from the Taito unit.
[ATTACHED IMAGE - CLICK FOR FULL SIZE]
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