Not Ranked
I too have done this, but on a 8800 RPM Datsun 510 with crank fired ignition. The holes were put midway between the plug towers directly in the plane that the rotor traveled in horizontally. Condensation, moisture and or humidity will cause cross firing within the cap. The holes are intended to allow the humidity to disipate from the heat of the distributor body/engine or that produced by the action of the sparks themselves.
Some of the later large diameter Ford distributor caps have an actual vent designed in the top near the center terminal, covered by a little black plastic cap that was held open by design to allow venting for the purposes described above. This may have merit in some climatic conditions but I can't say it ever benefited me, perhaps if there was more humidity.
Rick
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