Probably something to do with the automatic choke... probably to draw hot air thru the choke.

Check to see if there is a vacuum present at the port..

Does the car run ok without it? If so try putting a cap on it and see if it passes smog.

If not investigate farther.

I just moved from so cal and passing usually depends on whether you get a friendly tech. Just be polite and maybe talk about classic cars...

Also.. is the carb 5he factory one.. 5here are a lot of different models..

If the tech questions yiu about the plugged port.. tell him it was that way when you bought it.
Good luck

It is the port that goes to the dist vac advance on the 74's would think would be used as the same function on yours.