cb350k questions....

We tore apart RedQueen's carbs for the rebuild but ran into some issues. The mixture screw doesn't match up, so we'll be using the stock set. Also, there are some misc bits that I can't ID. I know where the pilot and main jets are but there's a 3rd spot that I'm clueless about:





Here's a pic of some dirty old carbs but they're "complete":

Any ideas on the mystery "hole" and parts?