The 500sel transmission is Mercedes part #126-270-37-01, or #126-270-08-00 or #126-270-59-01 or #126-270-49-01 and all are a 722.311 while the 420sel is Mercedes part #126-270-29-00 which is a 722.355 according to:
http://www.autoshoppingcenter.com/transmissions/MERCEDES400transmissions.html
You could also ask these people:
http://www.europeantransmissions.com/Tranisite/mercedes.htm
or
http://www.jie.com/Mercedes/mercedes_benz_transmissions.htm
It looks like they are different.