TD504 colour densitometer thumbnail guide:

Check that screen adjustmentents under diffuser is set to match aperture of diffuser disc in use.

Place yellow dot down. This is general purpose filter. Power unit up. Read clear edge of test target by depressing reading lever. Press zero button to zero display.
Read other target denisties. There are adjustments on the back, but no need to change them most of the time.

Garden variety uses are to read a b&w neg on yellow filter to determine the best paper contrast to print as a first guess for a neg, and to fine tune your film speed and development regime.

It is a heavy thing. I leave mine in one place in the adjacent laundry under a dust cover , rather than try to lift it on and off the dry side bench in my darkroom when I need to use it. Delicate expensive /nigh impossible to replace photomultipler tube; I have not bought a spare bulb, because if tube dies first this unit will get tossed.

I have a manual for it somewhere, and can scan it if the buyer feels they would benefit from it's knowledge.

I learned much more on how to best use it from a late 70's Kodak book on duplicating.

The filter barrel may be rotated to measure the R, B and G response of C-41. It is entirely mechanical and may be pulled out to check for occluded filters. There were are A and and M filter barrels, tailored to reading negs and transparencies, but I have not found a huge problem plotting curves for both types using my M (i think) filter set.

Useful for determining cross over from aged colour film or paper by doing a series of readings from a test neg shot at different bracketted exposures first time of a test target of white to black targets and plotting them on log/linear paper to see how the three curves respond. They should track each other with a constant space between them. Usually a second test is needed once expsoure is nailed to determine needed filtration, and maybe a third to finess developer activity for precise colour control.

In crossed curves the highlights usually end up crossing one another, and there is nothign that can fix that, so start dreaming up artisitic interpretations to use that aged stock up.

They are also used to calibrate development, exposure, and filtration when fine tuning transparency dupe film and interneg films, where there are careful parameters to tune up for best results.

In front of lens filtration can adjust the spacing between curves until it is right, with each curve so far from the next one.
Film speed/exposure can be determined by seeing that even the black target is not sitting on the toe, but just above it.
Development temperature/time/activity/replenishment rate can be tuned up my monitoring the slope of the three curves in colour, or one curve in b&w.