I recall there was an OEM condenser for the later W126 designed for R134-A. I used to have a part number but I gave up on my Air conditioning ever working effectively again so I did not pursue it.
I read constantly that the reason the ACs did not perform well with R134-A was poor condenser performance so I tried an experiment: I put the AC on high recirc and turned a water hose on the condenser full-blast. No noticeable difference in the output temperature. I also installed a generic parallel-flow condenser in a 1977 Rolls-Royce and not much if any noticeable difference.
I simply do not understand why these systems do not cool, yet those in new cars with the same refrigerant are most effective. I have threatened to remove an entire system from a car at the salvage yard and fit it into the 420SEL so I don't have to be hot and ashamed of the AC performance.

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Providing a home for these cars:
1951 Buick Special De Luxe
1977 Rolls-Royce Silver Wraith II
1970 Rolls-Royce Silver Shadow
1986 Mercedes-Benz 420 SEL
2005 Mercedes-Benz E 320 CDI
2012 GMC Sierra 1500 4X4
1927 Pierce-Arrow Series 80
1931 Pierce-Arrow Series 43
1926 Ford Model T coupe