Holley/Demon Carb stumble

I think @yellow rose got it covered.
The only explanation I've seen for how we got to race cars using low manifold vacuum enrichment came is from Tuner.
I'll dig it up and put it on the Power Valve thread.
But the short of it wasrelated to a lack of power valves that could survive the new fuel formulations.
More radical engines generally needed to richer earlier in the throttle. The original 3310s for the 396 Chevy had 10.5 and 8.5 PV openings. The Chrysler FSMs I've looked at show 8.5 PV for the Holley 4160s where they were used.
An engine that has good part throttle fuel distribution and efficiency will not need enrichment until much further in the throttle, maybe 6.5 PV or even 5.5 PV.