... worth checking that the 'shoulder' on the output shaft is below the
contact surface for the nut on the drop arm. If proud it'll be
impossible to get it tight. Assemble first with a washer with a hole
that will allow the end of the spline to pass through and no tab washer.
Tighten to seat the components. Remove nut and check as above. If OK
replace washer with tab washer. If the spline pokes through you'll need
to use a washer to prevent the nut bottoming.

I think that there was an issue along these lines, and may even have had
it myself but the memory is hazy. Perhaps someone else will remember.

Other silly things to look out for are burrs etc. on the splines (both
male and female) and the possibility of a wear step if the drop arm has
been rattling around for a while.

If the joint tightens properly the tab washer should be just a back-up
measure. There are some drop arms which do not have a good flat for the
bent over tab washer to seat on properly. Early tab washers were not
really big enough to do the job properly.