Sludge in bore trapping the piston and/or spring completely collapsed?

Try to move the piston in a bit to break any adhesion.
If it moves in but doesn't return to the start position, use a magnet retrieval tool to
remove the piston.

Edit: After closer inspection of the diagram...
There is a clip (item 71) that probably attaches to the rod (item 76) holds the assembly together.
You probably have to remove the whole tensioner to disassemble it, and replace the gasket (item 85) as well.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022