Good guess! I was trying to put it between the forend and the barrel...and what's more, I found an old spring, buried in crud where it should be, while I was exploring the recess and thinking about cutting out wood. I put in the new spring and washer because the old one was practically flat. The forend and barrel now go together.
I am left wondering how tight to make the inner band screw....
And thanks to all who helped.