I have the same problem on my '77 280E (W123).
In addition, one of the bars on the black plastic shroud that holds the metal belt tongue is broken.
I have been quoted an exorbitant price to replace the whole belt mechanism with a (non-MB) aftermarket item, which I really don't want to do.
Is it possible for a dealer to replace a spring in the mechanism (presumbly to speed up the 'return') and re-stitch in a new tongue assembly?
has anyone got experience of this matter?