That's it. At that point in time the federal emissions test the car had to pass involved a tailpipe sniffer on a warmed up idling car. I think at the factory they hand tuned that special idle circuit on each individual car, then put a dot of paint on it to 'lock' the adjustment in. This took care of the production variances on each car and made the idle clean enough so that they didn't need to include air pumps and other ugly emissions controls equipment like so many other early 70's cars.

A wideband O2 sensor is nice (I have one on my PV to help debug the DCOE's) but might be a bit of overkill on a D-jet system on a stock engine. It should be pretty darn close to proper all the time (at least close compared to what carbs do) unless there is a wiring or component fault. Probably the only tuning tool you need is pulling the plugs occasionally and looking at them, maybe applying a slight tweak to the fuel pressure from time to time. If you find that it needs much adjustment from stock at all then there is probably a bad sensor (MAP, water, air in order of influence).

Although once you've put an O2 sensor in it's another step closer to putting on a modern FI system like MS, with a feedback loop.
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I'm JohnMc, and I approved this message.