I haven't seen the top open, but I am fairly new to these myself. I have rebuilt 2 and haven't seen the side vents plugged up. I'm sure someone will be by with more knowledge than me to help you out. You can search 1904 in the straight six forum, or Mikes Carburetor Parts is a great source for info, always responds to emails as well.

Holley 1904

Holley 1904 Carburetor Parts