The picture you posted of the labels showing how to position the rings indicates that you have the correct forearm unless Browning messed up big time. I doubt that. I bet the mystery piece was used to make up for a weak spring. The new spring you are getting will be interesting to see. It may be a bit stronger or a bit longer. That would explain things. The kit should get your shotgun running right. I have replaced springs before, and none of my Auto 5 guns is quite as old as your gun is.