Two on left are A12 auto front, 70 4 spd rear,two on right are both A12 4 spd.I have found quite a few A12 without bowl vent but all were early
March production,all main body casting the same,69/70.Also,most A12 center's had black bowl vent arm.On the Corvette tri-power,the 67 had no bowl vent,68/69 had vent. As to whether it would be an issue,the Holley 4 bbl dual inlet center hung front bowl came with or without a vent depending on the application.