This is an infinium crossfader. Probably not the fader from the Octatrack, but still the same idea.
Please note the tiny silver "reflectors" or shields or hoods. They're similiar to your household burglar-alarm motion-detector. There is a printed plastic (encoder plate?) that glides between the two reflector/collectors. My best friend tried to be fly and put a little dab of oil on his Mackie D2 glider rails to get it to slide smoothly (an old Rane trick).The globs collected on the printed plastic piece, as well as pubes, dead-skin, and chocolate cake. Compressed air fixed it.
I had to clean mine recently. All better. If you try to blow air in the slot--be wary of compressed-air condensate/etc. Don't blow it while it's on.... I dunno. Have you ever turned your qwerty keyboard upside-down on your desk and shook it out?