Wasn't that the idea of puting thick cardboard rolled up inside the tailshaft???.....to stop noise???
The damper is for vibes.....as is the rubber mounted weight you find bolted to the extension housing.
There are 2 different dampers.....a thin one, about an 1" thick, and the big one, nearly 2" thick.
They tended to be found on wagons, Fairlanes and Fairmonts, some XR6's, utes & vans.....depending on transmission fitted.

A couple of posts explaining the purpose of the slip yoke damper:

gearbox-suspension-brake-driveline-f4/manual-driveshaft-how-essential-is-it-t58306.html

gearbox-suspension-brake-driveline-f4/quick-question-on-ef-el-tailshafts-t58617.html

Interesting. I have seen them on EF/ EL Auto & manual shafts, also seen EF & EL shafts without, both auto & manual. I have not yet seen them on EA-ED car's. I have 2 EB Xr8's with alloy tailshafts, manuals without these on the shafts, and my EB Ghia I6, it is currently running a manual gli tailshaft, without damper. The old auto tailshaft didn't have the damper either.

Cheers

ToranaGuy