I found some interesting info. These are 3 spring brands that make springs for the W126 along with their respective part numbers

intrax 50.1.013 380SE/SEL/SEC/300SD/500/560SEC NSL
50.1.022 420/500/560SEL NSL

eibach 2508-140 All SEC/SEL SL

h&r 29559 All SEC/SEL NSL
52759 All SEC/SEL SL

*NSL:No self leveling SL:Self leveling

Since we already know W126 springs fit the W123, we just need to find out which W126 has the closest weight to the W123 weight. Done that, we'll have a source of "performance" springs... Thoughts?

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1982 W123 300D NA Euro, 300K Miles, 4-speed manual, 3.46 rear diff, early W126 front brakes, Penta rims with 205/60R15 FH900's, custom fitted (by me) audio system, more to come into the money pit... Soon to be installed: Bilstein HD, R107 front discs and 4-piston calipers, HD Suspension springs, wagon front swaybar.