I was wondering the same? but it's only a drawing! found a nice colour film piece, maybe 2 seconds of 1940-41 of rows of B4's will try to get it on Youtube.
Ian, yes you'd need an oxygen mask, when I was driving!!!!!
It looks like 20 'Capstan' to me. Was the passenger about to light up ?
At that speed it would be tricky to light a cigarette.
Could it be one of those wooden blocks to hold a rifle butt, I realise there isn't a contemporary photo to prove it but this drawing is pretty accurate in every other way.
I agree Rob..it is highly likely that the artist worked from a photo and included an existing detail (as opposed to inventing one unneccesarily)..Could this mean all existing Big 4s are incomplete?.. ...Ian