[alfettaparts2]

You are missing the outside part of the spigot bush. Its a metal ring that pushes over the egg shaped thing. Most probably broken when the last person forced the tail shaft together without the bush being aligned.

 

[alfettaparts2]

Would the bush being broken cause the flex disc to fall apart?

Probably but I'm only guessing. Fair chance someone broke it forcing the two halves together as well. You need to be patient when joining the two halves, they should slide together with gentle effort, never any force.

If you chase up a replacement from the wreckers it will pop on with a bit of lube and a soft faced hammer.