These 80s era knobs are sourced from a Grass Instruments polygraph DC amplifier. They fit 1/4" inch / 6mm shaft and have an Allen head retaining bolt. I have several sets available and some of the knobs do have minor flaws here and there. Overall I consider these in very good condition for their vintage. They have a solid high quality feel and a nice weight to them. Please refer to pictures and feel free to contact us with any questions.