S&S SUPER B 1 7/8"

This early Super B Carburetor has the vent hole plugged?  Is it internally vented?  I see there's a passage that runs from one end to the other, inside.  Serial No. 3275.  Must be from 1975-1980.  Kinda cool with just the single throttle cable.  I could run this on the 80"er I build someday . . .