You might try a salvage-yard for an early 123 bumper.
Early W123s had bumpers with two channels molded on the inside. Inserted in these channels were 8 carriage-bolts (4 per shock) to attach to the shock mounting-plates. Also the rubber strip had a groove that was pressed over a horizotal rib molded to the outside of te bumper - a much better setup than the cheezy plastic clips used later.

Otherwise, I'd try somthing like what someone did on the thread 'my bumper fell of' listed here -
drill the bumper and shock mounting-plates to accept some stainless carriage bolts & nuts. I'd try to locate them where all the bolt-heads would be hidden under the rubber strip.
Instead of the flimsy washers shown in the other thread, it may be better to relocate the carriage bolts a bt farther apart, and use some heavy steel plates to re-clamp the bumper-shock mounting to the inside of the bumper.

Happy Motoring, Mark

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DrDKW

The earlier bumper will also require different mounting struts.

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'83 300D, 126K miles.

Actually, this is sort of similar to the setup my 85 has. But instead of decent size bolts there are these sort of shrimpy little tacking nail things. For now I've put some adhesive on it to hopefully hold it in place until I can make a better repair.

The rubber on the the other side has cracked as well and it's only a matter of time before that becomes a problem. I would think the rotting rubber is lurking out there in many W123s.

Thanks for the replies!

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85 300D 180k