Welcome. Maybe this will help,,,,, you can find more here,,
http://stevespages.com/page7b.htm
View attachment 18192
The Numrich schematic calls them the elevator spring and elevator spring follower. It seems there should be something under that elevator assembly to either hold it in position or align it somehow. Does the elevator work at all now? I have run into situations like that where it took three hands and a bible to hold everything in place until all the parts were locked in by the surrounding framework. Good Luck.
If you don't get an answer here I would give Havlin a call,(636) 937-6401. They are the pros when it comes to older Mossbergs.
Can you see any shiny spots where that elevator follower may have been riding when it was in use? Any wear areas at all to indicate where the spring may have been sitting? Might be a clue as to where it should go.