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