Thanks Fun Times for the drawings.
So it is a coolant sensor for the gauge.
For the ECU sensor, it is weird that they call it a 'switch' when in realty it is a thermistor that varies resistance with temperature. Mine is working well, I tested it in hot water, warm, and room temperature vs the chart.
So, the canister I see at the brass drain plug is a knock sensor. It is a bizarre place to mount it. On cars, that sensor is always mounted directly to the block.