Proximity sensors for C-slots are available as reed and electronic versions.
Electronic sensors are the preferred solution for all factory automation applications, especially with high cycle rates. The most universal switch family is the SMT-10M.
Reed switches offer an alternative for simpler applications with lower pulse rates, for example. Reed switches are universally usable as PNP, NPN and they represent the only possible way of implementing position feedback as a potential-free contact. The SME-10M is then the preferred standard solution.