I think the chap in the white coat standing behind the other man who is doing something to the car could be the same person who is standing on the extreme right in the assembled group photo provided by Dave and is likely to be the shop foreman.
To anyone who may be interested, I’m pretty sure now that the chap in the white coat and sporting the moustache is John Bull who was the Assembly Line foreman.
Still have no idea what the circles were for though !
The circles could be cutouts for pedals on a carpet. He could have been just sketching and trying something out as those white lines are very thick and you wouldn't be able to rub them off a canvas/duck material if he's drawing on a hood or something. Trimmers, including myself use white chinagraph pencils and tailors chalk to mark on dark materials, either on the face of the material or the rear but the lines are never as thick as that, they are fine and chalky, not as pronounced as that. I'm stumped!