I think it may have been earlier, very near the end of the war, in the later part of the war the blackout restrictions were relaxed slightly due to a huge number of road traffic accidents and because we mainly had air superiority in the UK (apart from V1s and V2s which couldn't see lights), the DU DP (dual purpose) mask with the flip up bottom half probably came first but these seem pretty rare, I don't know if the MU was fitted to completed machines before the end of the war as new machines at this time were being prepared for use in the far east. So the MU was probably retrofitted during a refit in safer places where the war had already moved on.
Years back I bought a completely original 1946 B31 which had been in a barn for decades...It was basically untouched from new...That had a DU42 headlamp fitted (and combination levers)....My 47 parts list only gives a Lucas part number for the headlamp and I haven't cross referenced that as yet or compared it to earlier lists...My 1947 M21 had the MU42 fitted and as far as I know was also original....More checking required to confirm obviously...Ian