The P size and location varies, generally speaking the smaller P started in 1968, went to the larger style sometime in 69 and remained through 71. Location and clocking varied, likely random. They do not run on just the primary exclusively, the front carb (secondary) also has an idle circuit criticle to proper fuel distribution. The front to rear intake port variable is typical of a dual plane intake design, it has to do with proper fuel distribution at various rpms the first design without the Pentastar was the 1966 and early 1967 design, the Pentastar was added early in the 67 model year.