They're the same!
RealOEM for a 3.6 US car:
01 RADIATOR 1 17111468085 $457.56
01 RADIATOR 1 17112226336 $457.56
RealOEM for a 3.6 UK car
01 RADIATOR A=650MM 1 17112226336 $457.56
FOR VEHICLES WITH AIR CONDITIONING (S530A=YES)
01 RADIATOR A=650MM 1 17112226336 $457.56
FOR VEHICLES WITH NATIONAL VERSION AUSTRALIA OR TROPICAL VERSION (L810A=YES)(S823A=YES)
01 RADIATOR A=650MM 1 17112226336 $457.56
See, same radiator for both versions. Same goes for 3.8
01 RADIATOR A=650MM 1 17112226336 $457.56
FOR VEHICLES WITH AIR CONDITIONING (S530A=YES)
01 RADIATOR A=650MM 1 17112226336 $457.56
FOR VEHICLES WITH NATIONAL VERSION AUSTRALIA OR TROPICAL VERSION (L810A=YES)(S823A=YES)
01 RADIATOR A=650MM 1 17112226336 $457.56
Dr Sakke, it would be handy for reference if when you next visit your dealer, you get those two part numbers.
Maybe that can shed some light on the mystery, as RealOEM says one radiator, the same 6336 part number for all 3.8 cars. (I checked 07/1991 01/1994 and 07/95 builds)
Maybe, maybe, maybe, early American cars used a bigger radiator as they only did 55mph and needed more cooling in their sunshine states as there was not much wind coming in through the grill at that speed?
Regards
Ivan