How Do I Change Main Jets?

[North Coast Cab]

I need to change the main jets in my carbs, but I don't get it. I assume this is where they are:

But when I remove the screw it has straight threads with a hole through them. It doesn't look anything like the jets I have and it doesn't look like anythig is in the hole.
Am I stupid? I must be missing something.

[North Coast Cab]

Did i mention they are PMO's?
Now I'm thinking they are in a different spot than the Webers. Any PMO experts out there?

[john walker's workshop]

PMO jets load in from the top, in a stack. the main jets are on the bottom of the stack, emulsion tube in the middle, air corrector on top.