Instrument cluster vacuum meter reaction time?

I don't see anything that resembles an orifice. It's got what looks like a pot metal plug in it. I've looked at with various angles under the microscope, and I'm not seeing how air is supposed to bleed past the plug. The plug can't be removed because it's been peened in. I'm not adverse to putting a hole in the plug, but I am concerned about keeping any debris from drilling it out of the bellow.