If you need a new one,
https://www.***************/search/product.aspx?sid=vqae1i55kc0lzd55y0yt3055&makeid=800016@Mercedes&modelid=1192968@240D&year=1979&cid=20@Engine%20Parts,%20Seals%20%26%20Gaskets&gid=5560@Oil%20Drain%20Plug
I have one of those bolt remover kits, comes in handy, but you'll destroy the bolt head in the process. Or you could use an extractor to suck out the oil.

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83 SD

84 CD