Sight holes allow you the ability to see the indicator lights even when the sensor is threaded into the mount. The sensor cable can still run back through your swivellink assembly for a clean look and protection from damage. The metric sensor mount SLM-8 mounts to SLM-2, SLM-290, and SLM-2-2XS. The SLM-8 fits Allen Bradley, Balluff, Banner, Baumer, Carlo Gavazzi, Contrinex, Datalogic, Di-Soric, HTM, IFM, Keyence, Leuze, Omron, Pepperl + Fuchs, Red Lion, Sensopart, SICK, Tri-Tronics, and Turck.