Where to connect a c-wire on a furnace with existing two wire mercury thermostat
TR is Common on your unit, Every 24 volt device will have a Common leg as well as the 24 volt hot leg, just as a car battery has pos and neg, common is akin to neg in this case.
Common is called this cause every 24 volt circuit terminates upon Common to complete the circuit.
The last pic shown is the "time on"   "temperature off" fan delay, the 2  lighter colored wires are low voltage, 1 id the heat circuits 24 volt hot leg  being W or white, the other being Common or C.
The gas valve has the same 2 24 volt leads as well,  without completing the circuit nothing will work,  every electric circuit requires 2 legs of power be it 1 hot leg  and neutral/Common or 2 hot legs as in a 230 volt circuit.
Google 40 VA HVAC transformer to see what they look like so you will be able to locate it.
Your 2 wire thermostat will have only red and white shown on my diagram