Electronic – the purpose of this two pin component

That's a selenium rectifier. Google "selenium rectifier" and there's loads of info, photos. They've been obsolete since the 1970s. Decades ago, they were cheaper than vacuum tube diodes, and semiconductor diodes couldn't handle higher voltages and currents. The parallel plates are indeed for cooling.

http://en.wikipedia.org/wiki/Selenium_rectifier

It's just a spring in a tube. The spring is anchored at only one end. When shaken, the other end of the spring touches the tube and completes the circuit.

The transistor functions as a simple amplifier so that the full LED current doesn't need to pass through the sensor.