This is the exhaust elbow. The exhaust elbow serves the purpose of cooling the hot exhaust gasses so they can run through the rubber hoses inside your boat without melting them.
In this photo you can easily see my numbering system and the "dot" for repeatability.
The brass elbow coming into the top side of the exhaust elbow is the injection point for the raw water (ocean or lake water) circuit. This is where the water that gets spit out your transom is injected into the exhaust system to keep it cool.
This temp, and the next one, are CRITICAL measurements. Exhaust elbows on short run time sailboat installed diesels tend to load up with rust scale and carbon until they block. A blockage of the exhaust elbow will cause over heating!