Sven, they are late-war for use as the blackout regulations were gradually restricted on the home front and in safe areas but full blackout still required near the front.
I believe that they were mentioned from late 1944 onwards.
As Rik says its a late type mask fitted near the end of the war,if you look closely there is one fitted to this late war 3HW,this photo was taken on the 17th May 1945 out side the Triumph factory at the hand over of the 400,000th WD motorcycle.
As it's for a late war bike and mine isn't I'm considering offering it for sale together with a DU42 shell. If anyone is interested you can pm me. I preferably sell it to someone who fits this one on his running motorcycle.
The Lucas L-DU42-DP black out mask is listed in the Triumph 3HW SPARE PARTS CATALOGUE 26 along with the engine change over number,this is the last contract book,it look like Triumph changed to the later lighting set up early in 1945.