Control knobs GN 723.4 can be combined with reference flanges GN 723.3 which offer the following advantages: - they are provided with a reference mark relating to the scale on the control knobs - they give additional support to the control shaft, - the version with a friction ring prevents unexpected movement through vibration. Scale and arrow on the control knobs are indelibly marked and easily legible. Besides the standard scale (Type S) they can be supplied with any type of graduation. Regarding design, numbering run, numbering position and numbering sequence of the scale please see the layout for scale knobs on the order sheet „How to order Graduations“