I installed a center arm rest in my 1991 240 today, and I wanted to mention / clarify a few things. I read one how to that said the seats need to be removed. I did not find that to be the case, just push them forward as far as they will go. A 16mm socket is needed not a 17 as was mentioned somewhere. Also I read that the center console needs to be removed??? not the case with my 1991.
Anyway it was a pretty easy installation, the big bolt goes in at a weird angle that threw me for a bit, try it without the bar in place.
The beige leather cover I bought off of eBay is not even close to the same color, nit a big deal for me but might to some.
The points I made above are for a 91 maybe other years are different and that could be the difference.

Did you take any pictures? I am a little fuzzy about the two eras - one uses just the two metal arms on the armrest, the other requires a metal frame inside the console. ??

I did not take pictures sorry. It was the two arm one that bolts where the seatbelts go, install was simple. It goes up and down by use of a smaller bolt that goes through the armrest itself and through both arms. I can take pictures of whats there is that helps you.