Hello Arno ,
these sensors are induction nearing sensors from Pepperl and Fuchs. They have all datasheets on the Webside available in many languages. My up/down sensor was labeled NBB4-12GM30-E2 . Switching distance is 4 mm , so I adjusted them very near to the sensing metal which gives the right position to turn the gangway in to resting position.
As you mentioned the big problem for the gangway is saltwater between the stainless passarelle and the rotating actuator. You see on my picture that I cut the white nylon-cover and pressed grease in each screw-hole . For this procedure all screws had to be loosened for 3 mm and fill one screw-hole with WD40 and later grease, fasten the screw and by the time grease will come out at the borders. I hope this will keep me away from buying a new rotating part for 6k.
The opacmare company was no help ! Now answer a lot of contacts by email.
The man who works for opacmare service in La Rochelle told me that they often change the hydraulic oil due to rusty ironparts from the rotary actuator and that these parts may block the mechanical switches.
Wolfgang SY ELISE Amel 54#162 La Rochelle