How to reassemble agitator/regulator plates on Vicon wagtail varispreader 800

[Moggytom]

Have a Bicon Wagtail varispreader 800 regulator plate rusted away took it apart and replaced but there is a black rubber/plastic like ring which fits the outside of regulator plate but if used will not go back together so obviously it is in wrong place it had fallen down loose before we dismantled the machine so could not see where it sits
Can anyone help?

 

[Dave W]

Does it have a sort of elongated hexagon centre?

 

[Moggytom]

The centre piece is definitely elongated rather than round the centre shaft will only go on if lined up correctly

 

[Moggytom]

Not sure I would describe it as hexagon but certainly the shaft is more oblong than circular