Carbs built after Jan 1, 1970 all generally have the same vent as the OP picture. It is correct that it was a design made for emissions control. Previous to Jan 1, they were not required. I’m sure it was an EPA thing. As to why it’s not working as it should direct from Holley, is a mystery. I restore six pack carbs now that Bill O has passed and have many carbs in the works right now. I’ve got some original carbs that are off of my AAR so I’ll inspect them when I go to the shop. I’ve done multiple sets of the early style bowl lately but I have a set in progress with the emission bowl so maybe I can check it out today. I did have a problem with the last emission bowl set and I used a new Holley vent arm. I had to bend it quite a bit to get it in the proper specs. I did not try and compare an original arm. Looks like I may have to do that. Be careful in bending that rod. It is made out of hard material and does not bend without a lot of force. You can mar the plating very easily, though.