Relatively low production car components like a Holley carb for a Z28 were batch built and stock piled for use as the cars were ordered, but they were banging out 4 barrell Rochesters (and carters under contract) day in and day out to meet production numbers. Its just not economical for a manufacturer to tool up everyday for something production numbers like like 30 carbs a day, but if they could bang out 1000 of them all at once and then wait a month before doing it again it becomes much more econimical for them. I think you'll find the more common the part, the closer to the cars actual assembly date the components got.