For anyone interested I have had a reply from MKS
"In exceptional cases, when the cap is loose or has fallen off, the bearing on the cap side is no longer fixed in position, which can lead to axial rattle.
In this case, tightening the will fix the bearing position and eliminate the rattle.
The tightening torque for the cap is 10 Nm."
I presume the missing word is 'cap'
I'll try when I can find my small torque wrench and the adapter for the Allen key
Happy New Year