As mentioned before , it was not designed with volume adjust in mind, the white plastic part (with a sealing o-ring at the bottom) is to make the air chamber ramp up sooner due to the reduced travel of the 29″ fork (650b no volume spacer + more travel)
Hopefully not damaged , most likely done the shaft bolt up on adjuster .
Remove the dial ,
loosen the foot bolt,
wind rebound to max (clockwise),
Refit shaft bolt ,
refit adjuster,
adjust rebound speed to suit .
On my second 2.0ltr petrol mondeo, 1st one got to 170,000 before i wrote it off, current one now done 180.000, petrol models have dual mass flywheels .
Both pre face lift 52 plate
2 shocks available , special van rc @£285 std (£345 push tuned) or
cane creek coil @ £485
If it is to replace a 5th air a new rear mount is needed as well