This is a dome top fine tuner knob with 6-40 (fine) threading for Kahler 2500 and 2520 Traditional fulcrum models. They screw into the 9417 fine tuner levers and are only available in black.
There were two types of fine tuners used on the 2500 and 2520 "Traditional" models. Either this dome top 8385 fine tuner with a 6-40 fine thread or a flat top 8394 fine tuner with a 6-32 course thread. Just inspect the fine tuner hole or your remaining tuners to see which one yours came with.
On another confusing note, this dome top fine tuner looks just like the dome top fine tuner used for CAM systems, but the CAM fine tuners all have 6-32 course threads. So if you need a fine tuner for a CAM system, see the dome top 8395 fine tuner or the flat top 8394 fine tuner (also used on FLYER models).