Bing Carburettor Question

Can anyone advise as to what the O rings at the bottom of the slide piston are for (part 22 in the diagram below) and if they are required ?



My 1992 R80GS did not have them fitted and just wondering whether it should have ?

moto-bins list them as :

CARBURETTOR LARGE O-RING AT SLIDE BASE
13 11 1 338 692
R80g/s | R80ST | 1981 - 1987
R80GS & PD | R80R & Mystic | 1988 Onwards
R100GS & PD | R100R & Mystic | 1988 Onwards

Any information welcomed

I think I read somewhere that it acts as a shock-absorber to stop the piston closing with a bump. But I am sure there will be others along shortly.

I think I’ve only ever seen them on later R100gs 40mm carbs.