Note:
ST50 & ST60 Depth Transducer connections are different. ST60 has 3 connections for Depth at the back of the Instrument. ST50 has 2 Depth connections at the Instrument plug where the black & screen cable cores are connected together.
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I'm not sure you can easily get a ST-60 Masthead unit now by itself. West Marine used to sell them, otherwise I think you'll have to go looking around. There were a bunch of listings on the web, but I'm not sure they aren't outdated?