Here is all I can tell you. The factory parts manual tells it like this.You're probably right, and I'm not saying your wrong, @pnora . But when I search epay using "Carter AVS 4966" the first 10 listings I see, all have the ECS internal float bowl vent. What are the odds all 10 of those carbs are wrong? I'm thinking really long odds.
1-The 4740 is listed for the 1970 model year with ECS system, It is referenced under 62 for components. Group 14-40-23 is for the external bowl vent arm This carb does not list a number for an arm.
2-The 4966 is listed for the 1971 model year with no reference to the ECS system. It is referenced under 102 for components. Group 14-40-23 is for the external bowl vent arm.This carb does list using the arm part number 3514509.
I can only tell you what the Factory manual shows. I have the hard text in the shop. You can download the manual at mymopar.