Well, I went back and read the whole thread; you've definitely got a replacement engine in your '90 300SE. Those are the dished pistons of the earlier models. Your engine should have 981 XXXX 126 stamped on it just in front of the right motor mount. I bet it says 983, which is out of an earlier 300E. Not that it matters at this point of course.

Those old timing chains are really tough. You never hear of one failing; I have over 250k on mine and the tensioner is still on the same "notch" it was 15 years ago. All they do is run in a big oval. And someone here once remarked that the camshaft on a single-cam straight six has the lobes arranged so that for every valve that is opening there is a corresponding one that is closing, resulting in opposing forces and very little effort required of the chain to turn the cam - interesting tidbit there.

Cheers,
DG