from the photo, the work machine appears to have a well worn collar judging by the amount of 'lipping'. Is the group handle still locking in at 90 degrees or does it turn much further? The coffee stains on one side of the seal seat seem to indicate leakage under the seal in this area.Rather than replacing the group collar, which could be rather expensive, you could try fitting a shim or two between the new seal and it's seat as per several previous posts in this section.
You may also need to remove some of the burred lip if you are finding difficulty in fitting the group handle up into place.
PS Is the shower screen retaining screw sheared off in the second photo?