It probably goes into an immersion pocket. It is a brass tube with a sealed end and a 1/2" BSP thread at the other end. The pocket screws screws into the cylinder/pipe, the sensor element goes into the pocket. You can remove, check, calibrate and change the sensor without draining down. Look on the Sontay website.
It looks like mains voltage; weird.
It may be a clamp-on job.
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