The later carbs have a solenoid on the carb top the earlier carbs do not have. Standard has a kit that contains the gaskets for both the early and late carbs.
I figured this out yesterday as I have my 76 carb and a pretty rough 79 as a spare. Took the 79 partially apart and stole the electric choke from it. Rebuilt the carb last night and today, rerouted my water lines, put the electric choke on, and tuned it. Unfortunately the Napa kit didn't come with a power valve, so I'll have to pick that up this week and put it in. The car already runs a lot smoother though.
The Napa kit also came with 2 identical gaskets for some reason. Did not have the later style one, the kit was specifically for 76 and older.