Pretty much everything is the same on an M40 and an M41, with the important exception of the tailshaft and its housing. On the M40 the shaft is short and splined to take the driveshaft flange. The housing is of appropriate length and has an oil seal in it. On the M41 the shaft is much longer, and has a different spline to engage the OD unit, as well as a cam for the OD oil pump. Insttead of a tailshfat housing, there is a coupling piece. made of cast iron, to connect the transmission and the OD. You can mix and match parts between the two, bearing in mind that there are two types, early and late, whose internals are not all interchangeable. To convert an M40 to M41, you will need the tailshaft and the coupler, as well as the OD itself. Unfortunately, major disassembly is required to get the tailshaft out of the donor M41.