AB Valve - Restrictor in Hose at Balance Tube?

On my 05/72 (with stock carbs and emissions stuff) , I removed the hose from the AB valve at the balance tube, and out fell a little circular metal piece that resembles a very thick washer with a very small hole in the center..... I never recall seeing one of these things before (but it's been decades since I have done this, too). I reassembled things as they were, then searched my factory parts catalog to find out what it is. I had no luck finding any description of it in the catalog, and could find no archived threads in the forum that discuss this.

I'm wondering if this is supposed to be there? Do I need it? What does it do? etc? Can anyone shed any light on this little (really hard to find when you drop it) part?

Thanks!