rear end drop - help

the rear end on one side of my 69er has dropped maybe 2cm.
its noticeable, but not dramatic. i first noticed a couple of weeks ago.
i feared a broken torsion bar and took it to one noted mechanic and he 'thought' it was but he didnt sound completely convinced and there was a lot of brow scratching going on. the car still has damping and the suspension seems to operate the same on both sides when manually pushing the car up and down and doesn't feel excessively hard on one side while driving to me.
a friend suggested the rubber on the outer end of the torsion bar has twisted and rotated with age and therefore giving the drop.

what the verdict here?... has anyone experienced anything similar?

US '69 2.0 911T, s-trim, sunroof.

thanks.

You may have some rust in the torsion tube. This used to be a major cause of big time rear end sag.