Using a MacBeth TD-504 transmission densitometer for reading B&W negatives

Has been years since I needed to use my MacBeth TD-504 transmission densitometer. Unfortunately misplaced my notes on using the densitometer. I am using it to read B&W negative densities. On the front of the densitometer is a circular rotatable knob that clicks into 4 positions. There is a colored dot for each position: red, yellow, blue, and green. Question is which position should the round knob be in for reading B&W negative densities and is the colored dot to be read at the 12:00 or 6:00 position?
thanks

Yellow dot, at the 6-o'clock position.

If in doubt, it's an amber (like Wratten #90) filter which gives you "Visual" response. Yank really hard and the filter set will come out and you can see which filter is going through the light path.

I find it a remarkable coincidence that the #90 is commonly used as a black and white viewing filter... there may be a scientific reason behind the choice of Wratten #90 for both the "Visual" channel of the densitometer and for viewing...