can it be wired to an alarm buzzer on a boat?

This does not have an alarm prior to shutdown. image.png

Will this work on an older Mercedes marine diesel engine, 1970s?

works with any engine that has a port for 1/2" pipe thread, Mercedes were metric, you would need to make your port threads to 1/2" NPT. Also, To work this gauge right you need a tattletale relay to shut engine off when temp gets to high.

Which one has the single wire terminal post to activate the shutdown? That is the one I need.

They're all the single wire. The last few digits are the length of the capillary tube. Its a 20t series (gauge mounting size 2"), 250 degrees max and the rest of the numbers are the length (6ft) and pipe thread size (1/2).

You can use on virtually any DC voltage. It just grounds 1 wire when tripped. It must be used in connection with a Murphy switch or you could wire through a relay using the n.c. side for normal operation and this will ground and open the circut.