Many electrolux vacuums have had a setup on them where it would measure the amount of suction passing through the hose. When it would drop below a certain amount, the door on the front of the vacuum would open and pop the (Full) bag out. My mom's electrolux had a dial that you could adjust the sensitivity of the bag sensor with depending on what type of dirt you were sucking up. With the sensor on 1, it was set for large dirt particles. Thinking that you would need lots of suction for large particles, the bag sensor was rather sensitive at this setting. At 6, the sensor was the least sensitive, and the bag sensor would pop open only if the thing was completely clogged.
I am thinking maybe this piece could be some type of adjustable bag thing like the Electrolux vacuums had.