I usually fit the inner cork washer behind the inner cover at the outer end of the first tube..That seals the cover from behind. I fit the other washer at the outer end of the tube that is inside the cases to seal that side.
To test the set up you have, slacken the footrest bolt completely so the cases don't have any pressure exerted on them. Remove the footrest as well if necessary to achieve this...Then reach behind the cover and test to see whether the inner spacer tube has any free movement axially, it shouldn't...You can just get to it by hand from underneath but you may perhaps need to use a long screwdriver or similar...
I believe the assembly drawing found in some BSA manuals is incorrect....Ian
