I was going to try to upgrade to a smart thermostat but the sites I saw online said thermostats always have an R or RC wire. This is a 1920 house with oil furnace (getting very expensive). The last time I tried to add a smart thermostat to a non-C wire unit at our old place I ended up having to give an HVAC company a whole paycheck.
With none of the other wires, your thermostat is just a switch connecting ‘heating’ with its return wire ‘RV for heating’. A smart thermostat will require it’s own power supply, be that a battery or separate added wiring.