Ha! I was just thinking that I wouldn't mind replacing my thermostat housing with one from an early W116 300SD that doesn't have the thermo vacuum valve switch on it.

I'd say something like JB Weld would be your best bet, if you get it level and smooth. Baking soda and vinegar with a brass brush seems to work pretty well for cleaning corrosion. You can probably even soak it in oxalic acid (wood bleach) or oven cleaner.

If you Google MTC 3829 (Mercedes part number 6172002415), you'll find what may be a compatible new thermostat housing, and apparently without the hole for the thermo switch, but it lacks the hole for the auxiliary fan temperature switch, so you'd probably have to use part of your old housing, if it's even still good. It's pricey at about $90, though.

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1967 W110 Universal Wagon, Euro, Turbo Diesel, Tail Fins, 4 Speed Manual Column Shift, A/C
1980 W116 300SD Turbo Diesel, DB479 Walnut Brown, Sunroof, Highly Optioned, 350,000+ Miles