Re: Holley baseplates
#1) Early GM 780 vacuum secondary, later service style as evidenced by the thicker corners.
#2) Over-the-counter 600 cfm emissions style carb. The hole in the center is a restricted vacuum feed. Might be off of a 6619 or similar carb.
#3) Appears to be from a 3310-1 carb--the blank boss at the pass. side rear can be drilled for a side style PCV like the original GM bases were. Some rebuilders/restorers modify these bases by cutting off the PCV tube boss on the rear and welding the hole closed, then they drill the side for the PCV. The right front corner can also be drilled for ported vacuum.
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