300SEL front suspension
 

I am getting a clunking noise when I hit potholes from the left side. Under examination there is a rod which goes at an angle and end up in the chassis end with a big joint. The joint on the left side seems worn compared to the right side. What is this mount called. The rod is at an angle

Is is easy to replace this item on the 126 style 300sel.

It's the guide rod joint, or track rod mount. It's a ball joint attached to rear of the guide rod/track rod. It's about $60-70 or so I think. It's really pretty easy. Check the DIY section, there's a complete write-up on it there. Might as well do both while you're in there. You can actually pull and repair it without compressing or removing the coil spring, but be advised the spring will push the control arm backwards, and you will need to push it back forward pretty hard to replace the guide rod. Just have a friend too help.
And, yes, the major sympton is a clunking over sharp bumps, especially with the brake lightly applied.

DG

Yeah, that's likely the problem all right. My advice is to go ahead and replace any worn front end parts at the same time for two reasons - First, you're already "in there" so you might as well, secondly the car's going to have to be aligned afterwards so if you wind up replacing a tie rod a month from now you'll have to pay for another alignment.