I would add that, from my experience, such meters often had a front "flap" which had a slit to limit light to the cell in broad daylight. For lower light the flap was raised, enabling full sensitivity. My guess is that the red index marks would then be used. Some meters also had a auxiliary, large cell which plugged in, such a cell would also have its own index mark, like the black and red on this meter.
I've had several of this sort of meters, IIRC all were reflected light only. An incident light meter of this vintage would typically have a translucent hemisphere, resembling half a ping pong ball, over the meter's cell. Google Norwood Director to see and example of such a meter.