If you're looking at this, you probably know what one is. If not,
look here. It's a long roll developing tank for 16 and 35mm film. Usually used for movie film, it can also take long rolls from high speed winders on cameras like the Canon F1, etc.
I have one that is just a bit dusty, but seems to be in perfect working order. It is a beautiful piece of bakelite and aluminum design, just the thing for the masters of antique technology who call APUG home.
$50 plus shipping, Satisfaction guaranteed.