Update ... Transmission drain plug is 5mm Allen ... was able to fully seat the allen in the plug and using a 16" breaker bar, that drain plug was not about to give up and break loose ... using heat from a heat gun made no difference ... was not able to get the drain plug out ... next step was to drop the pan and watch the fluid run all over my garage floor ... 5 of the pan bolts easily broke loose, the 6th pan bolt must have been in cahoots with the drain plug ... it was seized and would not move no matter what I did ... retorqued the bolts that did break loose, put away my tools and poured a cup of coffee

I talked with an independent Mercedes shop about 35 mile away ... they assure me they have seen this before and can take care of my problem ... so I will make an appointment and put it into their hands ... all of this just because I was trying to replace a transmission filter at 100k mile service ... I still love these cars