This looks like a transducer plug to me. About 1 1/4 inches (31mm); has an arrow on top. The smaller "plug" is about 1 9/16 inches (40mm). This isn't close to the size of the Airmar units I have had on my last two boats over 30 years, so maybe it was from an earlier on? Anyhow, if anyone needs one or can identify it as something different, let me know via PM.