Comment
The main differences are the domed (MU42) instead of the flat glass (DU42), the clip that holds the rim on the shell and the fact that MU42 headlamps were used post war. 
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Glass,glass seal, rim, rim clip and spring , rim clip slot in the shell, reflector, reflector mounting to rim and in some cases the bulb holder are all differences that can be found between the two types...Ian