I think Mercedes made a catalytic converter kit for the 113s, rather expensive as I recall. Others probably know more about it.
Aside from that, I've wondered the same thing. Some considerations are that a modern 3-way cat probably would not work well, but the older type that controls only CO should. Also, these cars run rich, and cats can run excessively hot in a rich exhaust stream. Usually cats are fitted close to the engine to heat up as rapidly as possible, but in our case, maybe further back would help prevent overheating the cat (i.e., a rich mixture makes more heat, but less direct exhaust heat might balance it out?). That would be a bit of a risk, I guess. A high-flow aftermarket type cat might be best, that might also tend to run cooler at the expense of being less efficient. Or maybe a cat with an air pipe as used on some 1970s/80s vintage US cars, but how to control the extra air? And, to keep things simple, one might want a single exhaust pipe instead of the dual pipes.
George Davis
'69 280 SL Euro manual
