Sounds like the balancer type ring that is found on some cars with manual transmissions, I don't know what the proper MB nomeclature for this part is though! If in the EPC you look up the 124.130 models after 1989 under the propellor shaft you can see a picture of it in the parts diagram #176!

To get to the transmission mount top nut you will at the very least have to disconnect the flex disk from the tranny yoke, then loosen the 46 mm driveshaft nut. Then you might be able to collapse the shaft enough to lever the end of the drive shaft off the tranny output shaft nub which will allow you to push the flex disk and driveshaft upwards enough to get at the tranny mount upper nut. If not you may also need to drop the tranny cross member to allow the tranny tailshaft enough drop to get at it. Worst case, disconnect the driveshaft at the differential and then the center bearing carrier bracket so you can pull the driveshaft backwards enough to clear the nut!

The nut is 19 mm and there is not much clearence to turn it, I have and use a wrench from and old BMW tool kit that works perfect for this job. Its a "dog bone style" that's got a 12 point 19 mm ring wrench with a very slim profile and thin ring, other end is similar just 17 mm. Good luck!