Generally, they added a catalytic converter, an oxygen sensor, and some plumbing to vary the mixture slightly at idle to keep the cat healthy. This was done with a little control unit wired to the oxygen sensor, and then to a frequency valve to vary the control pressure. I've also seen a charcoal canister to catch crankcase fumes.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe