No expert on Holley, but are we confusing types emission systems and types of vents too? CAP later CAS was the actual emission control of the exhaust. All States were CAS by 68. The Carbs both Carter and Holley that had a vent tube were for the ECS (Evaporative Control System) which was a requirement for CA only starting in 70. This was capturing vapors, both at carb, and fuel tank to burn in engine or return to tank as liquid. Having a bowl vent with rubber stop (normally open) was not part of ECS since it vented to atmosphere.

Frankly, I do not know why there was a bowl vent to atmosphere that started in 67 for CA CAP/CAS. The bowl vents started showing up on Carter AFB in 67 for the CAP/CAS CA only cars. Became standard for AVS in 68 since all states were CAS. But this was only a Chrysler thing as best I can tell. Plenty of other manufacturers using Carter did not use it despite having to meet all 50 state emissions in 68 and up. Later other manufactures used the internal vent lever to the tube that went back to the ECS.

Having a vent that captured vapors and routed back to tank, or burned through engine makes sense. The vent that went to atmosphere on a bowl that had other vents to inside of air cleaner doesn't make much sense to me, unless it was a pressure thing to ensure proper calibration with a dirty filter. Open with closed throttle, and closed when you accelerated to full throttle and got a pump shot. At least that is how carter carbs worked. Again a Chrysler thing mostly.