I think mine is a TD-504.
If it is like mine, there are two different 'barrel' options. They rotate around the photomultiplier tube, and may be pulled off to inspect, and if necessary blow dust off of the filters.
One is the M, for colour neg film reading, and one is A, for transparencies. Mine has only an M. I use the yellow dot for overall density readings. I don't recall if the dot goes up or down, but when the barrel is out, the filter colour versus the colur dot makes it obvious.
Under the translucent acrylic base plate there is an adjustable wire gauze light attenuation filter. Mine came set to '2', and the plate has a 2mm hole (I think), anyways, it works for me.
I fire it up, and the light comes on, and the the display shows a number. With yellow selected (for b&w), I push the measure taking little lever down, over the film rebate edge to take a film base & fog reading. I then press zero to zero the display, and then start taking readings.
If you are worried about accuracy, read a step wedge to confirm the gain is set properly. Mine was, and so I have never fussed with the gain knob on the back.
I have a full user manual somewhere. PM me if you feel you need it , and I can dig it up and scan it. I does not say heaps more than what I have related here.
