Vacuum advance information and myths

You are saying the correct distributor accommodates the timing mechanically at idle and does not benefit from MV?
Basically yes.
The initial timing of a pre-emisions or non-CAP engine provides good power, and idles smooth and strong.
The same engine with a CAP package used much less initial advance. The mechanical advance in the distributor makes up for that above idle rpm.

Pick an engine in the mid-60s that had both a non-emissions and an emissions reduction package on it and compare the inital timing and rpm. Then look at the mechanical advance of in the distributor of each. I've posted a number of these in graph form. Here's one

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Even more extreme change in initial was the CAP vs nonCAP 273 4 bbl here.

Some of the old mopars only have MV ports on the carbs.
Love to see them.
Last time we thought we found one it turned out to be ported. That was a 1966.