The difference is the OD on the end of the tranny. And M41 is the '5 speed' version with the OD, the M40 is the 4 speed version without the OD. It would work on an ES, but you would need to either get the matching front driveshaft section or lengthen yours about 11 1/2 inches. You would probably really want that M41, however. Volvos are a little undergeared for highway driving in the modern world without that 5th gear. Many people go to considerable effort to swap M40's for M41's. If you are swapping out an auto tranny by all means don't put in an M40, get an M41 instead.