They retain vacuum passage for distributor vacuum advance. This is for the pair of blocks.
Fully Adjustable Carburetor Metering Blocks 4 BBL 4150 4160 Holley Carb PAIR
(Pair) Carburetor Metering Rod Step-Up Springs Silver 8" Hg & Retainer
Billet Race Calibrated Carburetor Metering Block for Holley 4150 Carb Purple
Genuine Holley 108-90-4 Holley Carburetor Metering Plate Gaskets Non Stick 4160
Edelbrock Carburetor Metering Rod 1956; AVS, Performer
Edelbrock Carburetor Metering Jet 12474; Holley .074 Main Jet 1/4-32
