I agree with the previous post. The cam timing mark should only be used to rebuild the engine after an overhaul. If the car runs with the 5 degrees slack, then I'd check the timing with a strobe light and see if it lines up. I just recently rebuilt the m110 out of a 77 280e. If I'm not mistaken the m123 distributor drive is also linked by the chain.

Even then, my m110 had at least a degree of free rotation from chain slack with a new chain, tensioner, and rails.

I wasn't aware that it was possible to do a doublerow chain swap on the w123 body engines. I've heard of it on the w126 bodies for the early model m116 engines of the 380 sedans.