I think I figured it out. I think it goes between the 2 & 3 carbs and acts as the bottom anchor point for the throttle return spring.

Now one more part I'm missing... There a tube joining carbs 1 & 2 and 3 & 4, and carbs 2 and three have a angled brass fitting that is lightly pressed in. I pulled one out thinking there was an o-ring in there. The second one I don't remember pulling out, and this is the one that didn't get bagged. Somewhere along the line it came out, and I don't know where it is. Shit.