Alistair:

I got a new one from BusDepot but went camping three times
instead of installing it.  I finally did it today.

My '81 does not have any inner cover over the mechanism.
But interestingly, the new replacement seems better
and more robust in design.  At least more cushioning
when the door hits the end of the strap.  We'll see....

http://jones.colorado.edu/VW/VWdoorstrap2.jpg

Richard