This morning I wrote an MB parts vendor for guidance on what the correct replacement part # is. He sent me exactly the catalog listing I posted in post 10, except in English (attached). And to reiterate an earlier point, the grey bolt in the pictures many are so sceptical of is part number N304014010009, obtained from MB by Pelican.

Moreover, I have a trusted Mercedes-only indy mechanic nearby who has been in business since 1970, so diesel W123s were his bread and butter for the middle years of his career. He liberally dispenses technical advice to customers learning to DIY, but I don't like to ask too frequently, lest I strain the relationship. But I finally broke down and called him this morning as well. He gave me only the N304 part as the correct part, and said that these bolts were not supposed to be torque-to-yield bolts, and that some prior mechanic likely installed those in error. This didn't seem quite right to me, as I feel sure I have seen photos of these bolts in tutorials, and they looked like ones I pulled out. He did say there was a relevant VIN-split, but I didn't want to take too much of his time, so I didn't want to ask him what the part # was for the split.

I'm guessing the part # for the split is the A112 number listed in the catalog. I'm tempted to order it for science, but it goes for between 60 and 80 bucks. If I knew it was the correct part, I'd gladly pay for it, but I have no idea what to think now.

Ironically, I just completed a valve adjustment on my other 83 240, which is my daily driver, last weekend, and I should have looked at the head of the bolts on that engine while the valve cover was off. But that was before the N304 bolts had come in the mail, so this issue wasn't on my mind. I'll probably pull the cover this weekend just to see what's there.

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