Werner is probably not joking
I just replaced both rear axles on my 450sl and the seals on both were handed unless you can get the upgraded part. the actual shaft itself has small threads machined into it to return oil from between the seal and the bearing back through the bearing. if you use the wrong one it actually pumps oil into the space. I bought new upgraded shafts without the thread so it didnt matter which seal I used. Here is the relevant bit out of the 107 bible. EPC seems to be down at the moment so I cant check it for the transmission seal but if you have an older car with the treads on the shaft (and I cant check for now) you may have a problem if using an aftermarket seal (right size number or not)