The NMEA thing was a guess - it would need some form of physical connection unless bult into the assembly.
But yes, I do agree a rudder angle sensor is usally (and more usefully) attatched to the rudder, but if it's not that, then what is it????
I have no time to crawl the net at the mo, but would the manual maybe tell us? Whose steering is it? - I know Teleflex keep a lot of their books of words online.....